Lck is a membrane associated tyrosine protein kinases. The Lck gene has been shown to undergo rearrangement and overexpression in some murine lymphomas. In human studies, it has been demonstrated that the Lck gene is localized to a site in the genome which undergoes frequent chromosomal abnormalities in lymphomas and neuroblastomas. Resting T cells contain high levels of the Lck protein and mRNA both of which decline upon activation of the T cells. These and other observations sμggest that alterations in Lck expression may contribute to the pathogenesis of some types of neoplastic disease.