A gene from human chromosome 11p11.2 was isolated and was shown to suppress metastasis when introduced into rat AT6.1 prostate cancer cells. Expression of this gene, designated KAI1 , was reduced in human cell lines derived from metastatic prostate tumors. KAI1 specifies a protein of 267 amino acids, with four hydrophobic and presumably transmembrane domains and one large extracellular hydrophilic domain with three potential N-glycosylation sites. KAI1 is evolutionarily conserved, is expressed in many human tissues, and encodes a member of a structurally distinct family of leukocyte surface glycoproteins. Decreased expression of this gene may be involved in the malignant progression of prostate and other cancers.