A pair of twins, one of whom has chronic granulocytic leukaemia.

Material and Methods Blood was collected from the twins, one sib, and their parents by venepuncture (Fig.). Blood groups were determined by standard methods at the Human Genetics Unit, University of Wesstern Australia. The antisera used were anti-A, en l-A, anti-B, anti-AB, anti-C, anti-c, anti-D, ar,.;-E. anti-e, anti-Cw, aniti-Ew, anti-ET, anti-V, anti-M,anti-N., anti-S, anti-s, anti-Mia, anti-Vw, anti-Mg, anti-PL, anti-K, anti-k, anti-Kpb, anti-Kpa, anti-Lea, anti-Leb, anti_Fyb, anti-Lub, anti-Xga, anti-Vel, anti-Wra., anti-Dia, anti-jsa, anti-Jka, anti-Jkb, and anti-Fya. The Gm and Inv types were done by courtesy of Dr. Arthur G. Steinberg. Transferrin and haptoglobin types were determined by vertical starch gel electrophoresis by the method of Smithies (I959). Serum alkaline phosphatase types were examined by a modification of the method of Arfors, Beckman, and Lundin (I963), as previously described (I. D. Scott and M. K. Scott, I966, unpublished observations). The parents, the twins, and the younger sister were carefully interviewed concerning their family history, and a standard questionnaire was completed for the twins.