The activity of natural killer (NK) cells is regulated by members of multiple receptor families that recognize class I MHC molecules, such as the killer cell inhibitory receptor/leukocyte immunoglobulin-like receptor (KIR/LIR) family and the C-type lectin superfamily. The Ly-6 alloantigens represent a family of phosphatidylinositol anchored proteins that play a role in the process of T lymphocyte activation. Ly-6 expression is commonly induced on T and B lymphocytes after activation by mitogens. Ly-6A/E is a phosphatidylinositol-linked transmembrane member of the Ly-6 family that is produced in humans after 4 weeks of age. ©-interferon and TNF regulate Ly-6A/E at the level of transcription and mRNA stabilization in certain thymocytes and T-cells.