It is not unheard of in the history of science that two sets of observations are incompatible and that eventually only one set proves replicable. This may be the case with X-chromosome markers in bipolar illness. Although my friend and colleague Professor Mendlewicz is puzzled at my letter, it offered a fair presentation of the current status of the controversy in this area. The first problem, inconsistency of the Xg antigen and color blindness linkage to the same marker, is no longer a subject of controversy as a result of the work of Mohandas et al,1 which clearly links Xg to steroid sulfatase on the short arm of the X chromosome, whereas protan and deutan color blindness and G6PD are in a small region on the distal end of the long arm of that chromosome.
The second problem is that the strikingly and consistently positive linkage series of color