Murine Ly-6 molecules are a family of cell surface glycoproteins which have interesting patterns of tissue expression during haematopoiesis from multipotential stem cells to lineage committed precursor cells, and on specific leucocyte subpopulations in the peripheral lymphoid tissues. These interesting patterns of tissue expression suggest an intimate association between the regulation of Ly-6 expression and the development and homeostasis of the immune system. Ly-6 molecules are low molecular weight phosphatidyl inositol anchored glycoproteins with remarkable amino acid homology throughout a distinctive cysteine rich protein domain that is associated predominantly with O-linked carbohydrate. These molecules are encoded by multiple tightly linked genes located on Chr. 15 which have conserved geneomic organization. The in vivo functions of Ly-6 molecules are not known although in vitro studies suggest a role in cellular activation. This review will summarize our understanding of Ly-6 with regard to tissue expression, molecular structure, gene organization and function.