Chicken MCAM, a member of the immunoglobulin superfamily of cell surface receptors, consists of the V-V-C2-C2-C2 Ig domains. It is a single chain glycoprotein with three mRNA splice variants. One has a short cytoplasmic tail, another has a long tail, and the third seems to lack transmembrane and cytoplasmic regions. It is highly homologous to gicerin, a molecule involved in neurite outgrowth and Wilm’s kidney tumor progression in the chicken, and to the human melanoma progression molecule MUC18. Chicken MCAM is expressed by c-kit+ hematopoietic progenitor cells in embryonic bone marrow, thymocytes, and capillary endothelial cells. MCAM-expressing cells sorted from bone marrow are enriched in progenitors capable of differentiating into T cells when transferred into the appropriate thymic microenvironment. MCAM has been proposed to be involved in cellular adhesion and homing processes.