Human cloning

Human cloning is the production of a person genetically identical to an existing individual, almost always understood to mean somatic cell nuclear transfer into an egg. Nuclear transfer has produced live offspring in more than twenty mammalian species, including primates, and has produced human embryos and stem cell lines. No human pregnancy from it has been reported anywhere, and the reasons are as much about a collapse in motivation as about law or technique.

§Two things called cloning

Reproductive cloning aims at a live birth. Therapeutic cloning, more precisely called nuclear transfer for stem cell derivation, aims only at a blastocyst from which embryonic stem cells genetically matched to a patient can be taken. The two share every step up to embryo transfer and diverge completely after it.

The distinction mattered enormously between about 1998 and 2007, when patient-matched stem cells were the main scientific argument for permitting nuclear transfer at all. It stopped mattering when Induced pluripotent stem cells arrived. Reprogramming a skin cell with the Yamanaka factors produces patient-matched pluripotent cells without an egg, without an embryo, and without the ethical objection that dominated a decade of legislation. Therapeutic cloning was rendered largely obsolete within a few years of being legalised in the places that legalised it.

§How nuclear transfer works

An unfertilised egg is held with a pipette and its own chromosomes removed. A somatic cell such as a fibroblast, a cumulus cell, or a lymphocyte is placed under the zona pellucida and fused with the egg by an electrical pulse, or its nucleus is injected directly. The reconstructed cell is activated chemically or electrically to mimic fertilisation, cultured to the blastocyst stage, and either transferred to a surrogate or used to derive stem cells.

The hard part is neither the micromanipulation nor the activation. It is that the egg cytoplasm must, within hours, erase the epigenetic configuration of a differentiated cell and impose that of a totipotent zygote. This is the same reprogramming that underlies Epigenetic reprogramming more generally, but performed by unidentified maternal factors on a schedule nobody controls. It fails most of the time. Embryos that survive frequently carry aberrant methylation at imprinted loci and abnormal placental development, producing the pattern known in livestock as large offspring syndrome.

1. 1996
   Dolly
   Ian Wilmut and Keith Campbell's team at the Roslin Institute produces a lamb from an adult mammary cell nucleus. Hundreds of reconstructed embryos yield a single live birth.
2. 1998–2005
   Species accumulate
   Mice, cattle, pigs, cats, and in 2005 the first dog are cloned. Commercial pet and livestock cloning follows.
3. 2004–2006
   The Hwang fraud
   Claims of human stem cell lines derived by nuclear transfer are published and then retracted as fabricated; the cloned dog from the same laboratory proves genuine.
4. 2013
   Human stem cell lines
   Shoukhrat Mitalipov's group derives verified human embryonic stem cell lines by nuclear transfer, using caffeine to stabilise the spindle.
5. 2018
   Primates
   A Shanghai team produces two long-tailed macaques from fetal fibroblasts, aided by a histone demethylase and a deacetylase inhibitor to relieve epigenetic barriers.
6. 2020s
   Conservation cloning
   A black-footed ferret and a Przewalski's horse are cloned from decades-old frozen cell lines to restore lost genetic diversity.

§Dolly and what she showed

The 1997 report of Dolly's birth overturned a settled assumption that differentiation was irreversible in mammals.¹ The efficiency was poor: a few hundred reconstructed embryos produced one lamb. That ratio has improved but not transformed in the decades since.

Dolly was euthanised at six after developing a lung tumour caused by a retrovirus endemic in housed sheep, and her arthritis prompted speculation that clones age prematurely. Her telomeres were shorter than expected for her age. A later study of cloned sheep derived from the same cell line found them ageing normally into old age, which weakened the premature-ageing claim without settling the underlying question of how nuclear transfer interacts with the mechanisms described in Telomeres and telomerase.²

§Primates, and why humans are different

Primate cloning resisted for two decades. Part of the reason was technical: in primate eggs, spindle-associated proteins are removed along with the chromosomes during enucleation, disabling the reconstructed cell. Modified enucleation methods and chemical stabilisation of the spindle addressed this, and in 2013 human blastocysts and stem cell lines were produced.³

Live primate births required more. The 2018 macaque work added treatments that relieve epigenetic barriers, including messenger RNA for a histone demethylase and an inhibitor of histone deacetylases.⁴ Even then, only embryos made from fetal fibroblasts produced surviving offspring; attempts using adult cells failed. Two live macaques resulted from a large number of transferred embryos and surrogate pregnancies.

That last fact is the honest state of the technology. Cloning a human from an adult cell would mean many surrogate pregnancies with a high rate of loss, malformation, and neonatal death, in a species where each of those events involves a person who has consented to something and a child who has not. No reputable body regards this as an acceptable risk profile, and the argument does not turn on symbolism.

Claims and their status

Several groups have announced human cloning attempts or successes since 2001, including a religious organisation that claimed a birth in 2002. None provided verifiable evidence. As of 2026 no cloned human birth has been documented.

§A clone is not a copy

Public discussion has consistently overstated the relationship. A clone is a delayed monozygotic twin, and one raised in a different uterus, a different family, and a different decade. Identical twins share a prenatal environment and a birth cohort and are therefore more alike than a clone and its progenitor would be.

Nor is the genome identical in the strict sense. Mitochondrial DNA comes from the egg donor rather than the nuclear donor, so a clone made with a stranger's egg has a different mitochondrial genotype — the same asymmetry that makes Mitochondrial replacement therapy a heritable modification. Somatic mutations accumulated in the donor cell are carried forward. Epigenetic marks, X-inactivation patterns, immune receptor repertoires and neural wiring all differ.

The identity question this raises is philosophically thinner than it looks. Nothing in Personal identity and continuity suggests that genetic identity constitutes personal identity; a clone is a distinct person with a distinct psychological history from the first moment. The real ethical concerns are about the expectations placed on a child created to resemble someone, which is a familiar problem in an unfamiliar form.

§Law and ethics

Reproductive human cloning is prohibited by statute in most countries with an established biotechnology sector. The Council of Europe adopted a protocol banning it in 1998; a United Nations declaration in 2005 called on states to prohibit cloning practices "incompatible with human dignity", passing by a divided vote and carrying no binding force. The United States has no federal statute; the Food and Drug Administration asserts jurisdiction over the procedure and a number of states ban it outright, an arrangement that resembles the gap analysed in Governance of human genome editing.

The ethical literature on cloning is unusually well developed because it preceded the technology's plausibility. Leon Kass's argument that widespread repugnance can carry moral information was made about cloning first, and remains the most cited statement of the position surveyed in Bioconservatism.⁵ Against it, critics argued that repugnance had also attended IVF and organ transplantation, and that the specific harms alleged, namely confused lineage, instrumentalisation, and loss of genetic uniqueness, either did not follow from the technique or already occurred in accepted practices.

What ultimately blocked reproductive cloning was neither argument nor law but the absence of a use case. For infertility, Embryo selection and donor gametes work better; for genetic disease, Human germline editing and selection address the cause; for replacing a dead child, the technique does not do what the bereaved parent wants. The one domain where nuclear transfer remains actively useful is animal work, including livestock, the conservation cloning of species with collapsed genetic diversity, and the edited pigs used in Xenotransplantation and by companies pursuing De-extinction.

If a human is ever cloned, the most likely route is not a fertility clinic but a well-funded private effort in a permissive jurisdiction, and the most likely reason will be that someone wanted it done. Whether the efficiency and epigenetic problems can be reduced to a level that would make the attempt survivable is an empirical question that nobody has an ethical way to answer.

§See also

• Induced pluripotent stem cells
• Epigenetic reprogramming
• Mitochondrial replacement therapy
• Human germline editing
• De-extinction
• Personal identity and continuity
• Bioconservatism
• In vitro gametogenesis

§References