Cell adhesion molecule that mediates both heterotypic cell-cell contacts via its interaction with CD6, as well as homotypic cell-cell contacts. Promotes T-cell activation and proliferation via its interactions with CD6 (By similarity). Contributes to the formation and maturation of the immunological synapse via its interactions with CD6 (By similarity). Mediates homotypic interactions with cells that express ALCAM (PubMed:24740813). Mediates attachment of dendritic cells onto endothelial cells via homotypic interaction. Inhibits endothelial cell migration and promotes endothelial tube formation via homotypic interactions (PubMed:23169771). Required for normal organization of the lymph vessel network (PubMed:23169771). Required for normal hematopoietic stem cell engraftment in the bone marrow (PubMed:24740813). Plays a role in hematopoiesis; required for normal numbers of hematopoietic stem cells in bone marrow (PubMed:25730656). Promotes in vitro osteoblast proliferation and differentiation (PubMed:25730656). Promotes neurite extension, axon growth and axon guidance; axons grow preferentially on surfaces that contain ALCAM (By similarity). Mediates outgrowth and pathfinding for retinal ganglion cell axons (PubMed:15345243). Homodimer (By similarity). Interacts (via extracellular domain) with CD6 (via extracellular domain) (PubMed:9209500, PubMed:16914752). Homodimerization and interaction with CD6 involve the same region and cannot occur simultaneously. The affinity for CD6 is much higher than the affinity for self-association. Interacts (via glycosylated extracellular domain) with LGALS1 and LGALS3. Interaction with LGALS1 or LGALS3 inhibits interaction with CD6. Detected at the immunological synapse, i.e, at the contact zone between antigen-presenting dendritic cells and T-cells. Colocalizes with CD6 and the TCR/CD3 complex at the immunological synapse. Detected on brain motor neurons, in differentiating retinal ganglion cells and in adult retina (PubMed:15345243). Detected on leukocytes and on lymphatic endothelial cells (PubMed:23169771). Detected in spleen B cells and T-cells (at protein level) (PubMed:9209500). Detected in adult brain and embryonic spinal cord (PubMed:15345243). Expressed at high levels in the brain, and lung, and at lower levels in the liver, and the kidney, as well as by activated leukocytes (PubMed:9209500). The CD6 binding site is located in the N-terminal Ig-like domain. Glycosylated. Mice are born at the expected Mendelian rate, are viable and fertile and display no obvious external phenotype. Unlike wild-type mice, that have tightly fasciculated and smooth nerve bundles, mutant mice have more loosely bundled nerves with many single axons extending out of the main nerve. Eyes from mutant mice display a variable degree of retinal displasia (PubMed:15345243). Besides, lymph nodes from mutant mice display reduced weight and cellularity, but appear otherwise normal (PubMed:23169771). Mutant mice have only half of the normal number of hematopoietic stem cells in their bone marrow (PubMed:24740813, PubMed:25730656). Survival of lethally irradiated mice that receive bone marrow from mutant mice is impaired, due to impaired homotypic cell-cell attachment, impaired engraftment and proliferation of mutant hematopoietic stem cells (PubMed:24740813). Mutant mice are larger and heavier than wild-type and have increased bone mineral density (PubMed:25730656). Mutant spleen has an altered leukocyte composition, with reduced numbers of CD4(+) and CD8(+) T-cells, B-cells, dendritic cells, neutrophils and macrophages, but no change in the total leukocyte number. Their lungs display reduced numbers of lymph vessel and blood vessel endothelial cells, but no difference in lung weight. Lymph vessels in mesentery and diaphragm are more densely interconnected and show a decreased level of hierarchical vascular organization in mutant mice (PubMed:23169771).