s and chronic lymphocytic leukemia (CLL). 2) Regulation of lymphocyte development, especially B and T cell development. 3) Cis and trans-acting elements regulating the variation of ???-gal expresstion in populations of lymphoid cells infected with lacZ recombinant retroviruses in which various viral and cellular promoters drive lacZ. 4) Cell lineage studies using introduced lacZ rDNA retroviruse