A single recessive gene, lpr, induces an autoimmune-lymphoproliferative syndrome in several strains of mice. The lymphoid organs of lpr/lpr mice contained cells with increased amounts of myb RNA, which codes for a protein found in the nucleus. A similar human lymphoproliferative disorder also had an increase in c-myb expression. Mouse T cells induced by mitogens to proliferate did not express large amounts of myb RNA, indicating that marked myb expression is not a general feature of lymphocyte activation and proliferation.