The Ly-6 (Lymphocyte antigen 6) alloantigens represent a family of phosphatidylinositol-anchored proteins that play a role in the process of T lymphocyte activation. Ly-6 family members share amino acid homology throughout a distinctive cysteine rich protein domain that incorporates O-linked carbohydrates. During hematopoiesis, murine Ly-6 molecules have unique patterns of tissue expression, from multipotential stem cells to lineage committed precursor cells, and on specific leukocyte subpopulations in the peripheral lymphoid tissues. Ly-6H (Lymphocyte antigen 6H), also known as NMLY6, is a 140 amino acid lipid-anchored membrane protein that belongs to the Ly-6 family. Highly expressed in brain with lower expression in pancreas, testis, colon and small intestine, Ly-6H participates in the activation of T lymphocytes. Overexpression of Ly-6H is detected in certain acute human leukemic cell lines (such as MOLT-3), suggesting a possible role for Ly-6H in carcinogenesis.