The Ly-6 complex is a series of genes found on chromosome 15. These genes code for a number of different proteins that can be used as surface markers. The family members vary in their biologic expression and have been shown to be involved in cell signaling and cell adhesion. The structure of these proteins includes a motif known as the LU domain that has three loops comprised of disulfide bonds. These bonds are formed by 8 to 10 cysteines that can cause differences in the length of the loops as well as the sequences at each tip. There are 11 known Ly-6 genes on murine chromosome 15 that code for different proteins. These family members, excluding secreted Ly6/Plaur domain containing 1 coded by the Slurp1 gene, are attached to the cell surface by a GPI anchor near the C terminus. The structure of these proteins may play a role in transmembrane interactions, and downstream signaling cascades. Ly-6 proteins have been widely used as differentiation markers on hematopoietic cells. The ability to isolate and express specific Ly-6 antibodies through hybridoma technology has allowed researchers to identify unique proteins. These proteins are expressed on subsets of immune cells at different stages of development, such as T cells, B cells, monocytes, granulocytes, and macrophages. The HK1.4 clone is widely used to activate T lymphocytes.