Complementary DNA encoding the human CYR61 protein was isolated from human embryonic tissues and mapped to chromosome 1p22-p31. We show that CYR61 encodes a 381 amino acid protein rich in cysteine and proline residues that is strongly conserved with the mouse homologue. Sequence analysis reveals the presence of several distinct protein domains which confer a mosaic structure to this protein and makes human CYR61 a member of a recently described growth regulator family that includes several proto-oncogene products. From our results we hypothesize that this new immediate early gene may play a role in cell commitment during embryogenesis and more generally in the control of cell proliferation.