A normal blood donor has been encountered whose red cell grouping tests showed a mixed cell population of CDe/cde, Fy(a+) and cde/cde, Fy(a—) cells. A family study revealed that the cde/cde, Fy(a—) cell population could not have arisen through a normal genetic pathway and reasons are given for suggesting that a gene mutation had occurred. As both Rhesus and Duffy genes are affected, it is concluded that the mutation has modified a common gene involved at some stage in the development of both blood group antig