B-CLL is the most prevalent leukemia in the Western hemisphere, accounting for ~25% of all leukemia's (1). This disease occurs virtually exclusively in the aging population, with the median age of diagnosis ranging between the mid 60s and the early 70s. Indeed, its occurrence before the age of 50 is quite unusual. This increase in occurrence with age is not unique to B-CLL; rather, it is characteristic several B cell lymphoproliferative disorders (e.g., non-Hodgkin's lymphoma, multiple myeloma). Gender and race also influence the development of B-CLL. Thus, the ratio of men: women is ~2:1 and the prevalence is increased in Caucasians. The rate of occurrence of B-CLL among Asians is significantly lower than for Caucasians and this does not increase with immigration to the West. DNA sequence analyses performed in our laboratory and in those of others indicate that B-CLL cells from unrelated patients share Ig V gene characteristics. These include the use of selected genes, the association of these genes with certain D and JH gene segments that code for unique CDR3 motifs, and the occasional occurrence of highly similar VHDJH + VLJL pairs. In ~50% cases, these rearranged genes are mutated, whereas in the others mutations are infrequent; this difference is related to the VH gene family used by the B-CLL cell.