Sialomucin core protein 24 also known as endolyn or CD164 (cluster of differentiation 164) is a novel 8- to 9-kD mucin-like molecule expressed by human CD34+ hematopoietic progenitor cells. Isoform 1 and isoform 3 of CD164 are expressed in hematopoietic and non-hematopoietic tissues. Isoform 1 is expressed by prostate cancer tumors and prostate cancer cell lines. The expression is greater in bone metastases than in primary tumors. Expression in osseous metastasis is greater than that in soft tissue metastasis. Isoform 2 of CD164 is expressed in the small intestine, colon, lung, thyroid and in colorectal and pancreatic adenocarcinoma. Isoform 4 is expressed by both hematopoietic progenitor cells and bone marrow stromal cells. CD164 belongs to the CD164 family. The cluster of differentiation (cluster of designation) (often abbreviated as CD) is a protocol used for the identification and investigation of cell surface molecules present on white blood cells initially but found in almost any kind of cell of the body, providing targets for immunophenotyping of cells. CD164 may play an important role in prostate cancer metastasis and the infiltration of bone marrow by cancer cells. CD164 promotes myogenesis by enhancing CXCR4-dependent cell motility. This protein positively regulates myoblast migration and promotes myoblast fusion into myotubes. CD164 may play a key role in hematopoiesis by facilitating the adhesion of CD34+ cells to bone marrow stroma and by negatively regulating CD34+hematopoietic progenitor cell growth.