Sir

In his engaging Commentary “IVF and the history of stem cells”1 Bob Edwards gives me undue credit in asserting that I produced the world's first mouse chimaera. Unquestionably, it is Krzysztof Tarkowski who should be credited with this. He aggregated mechanically denuded mouse morulae in pairs to obtain giant blastocysts, some of which developed normally to term following transfer to uterine foster-mothers2. Although perinatal losses were inexplicably high, seven of nine available young showed a mixture of pigmented and unpigmented cells in their eyes, in accordance with the genotypes of the morulae that were combined3. Subsequently, Beatrice Mintz introduced various technical improvements that enabled chimaeric mice to be produced routinely4.

My contribution was to show that similar chimaeras could be obtained by transplanting cells between blastocysts5. This was a refinement that enhanced the value of such organisms for studying development6 and was later adopted as the standard way of obtaining germline transmission of genetically modified embryonic stem cells.