CD5L (CD5 molecule-like), also known as API6, PRO229, Spalpha or SP-ALPHA, is a 347 amino acid secreted protein that belongs to the scavenger receptor cysteine-rich (SRCR) family of leukocyte regulating proteins. Expressed in bone marrow, spleen, thymus, lymph node and fetal liver, CD5L is thought to be involved in regulating the immune system via binding to peripheral monocytes and mediating their activation and overall survival. CD5L has three cysteine-rich domains and, in addition to its role in the immune system, may function to inhibit apoptosis and promote macrophage survival.