We were able to use our stored sera for epidemiological surveys and in a short time accumulated a considerable amount of information on the worldwide distribution of Au. One hypothesis stated that, although Au may be rare in normal populations, individuals who have Au are more likely to develop leukemia than are individuals who do not have the antigen. That is, there is a common susceptibility factor which makes it more likely for certain people both to have Au and to develop leukemia. We also suggested that Au might be related to the infectious agent (virus) which is said to be the cause of leukemia. A corollary of the susceptibility hypothesis is that individuals who have a high likelihood of developing leukemia would be more likely to have Au. Young Down’s syndrome patients are more likely to develop leukemia than are other children; estimations of the increased risk vary from 20 to 2000 times that of children without Down’s syndrome. I had in 1964 moved to The Institute for Cancer Research in Philadelphia to start its Division of Clinical Research. While there we tested the sera of Down’s syndrome patients resident in a large institution and found that Au was very common in this group ( ~ 30% were Au positive); the prediction generated by our hypothesis was fulfilled by these observations, a very encouraging finding.