Human chimaera detectable only by investigation of her progeny

A CHIMAERA can be defined as an organism whose cells derive from two or more zygotes 1 . In man, two categories of spontaneous and permanent chimaeras are recognised: blood chimaeras which arise from the exchange of blood-forming cells between dizygotic twins, and dispermic chimaeras which are the result of the fusion of two zygotes and of their growth into one body (for review see ref. 2). Blood chimaeras are usually identified because of blood-grouping problems caused by the mixture of different erythrocyte populations, while dispermic chimaeras who often present sex abnormalities are mainly detected by cytogenetic studies, although they also carry a mixture of different blood cell populations. In contrast to these typical cases, we now report a unique human chimaera who shows no mixture of blood cells as far as the genetic markers investigated are concerned, and who is only detectable by the investigation of the blood of her progeny (family Eiw., Fig. 1).