The macrophage inflammatory protein-1 alpha (MIP-1 alpha )/CCL3 and-1 beta (MIP-1 beta )/CCL4 are highly homologous CC family chemokines that were originally co-purified from medium conditioned by an LPS-stimulated murine macrophage cell line. Mouse MIP-1 alpha and MIP-1 beta are encoded by their respective single-copy genes. In human, additional non-allelic MIP-1 alpha and MIP-1 beta genes also exist.The two human MIP-1 alpha genes arise by duplication/mutation. They code for MIP-1 alpha isoforms CCL3/LD78 alpha and CCL3L1/LD78 beta, which share 94% amino acid (aa) sequence homology. Whereas the human CCL3/LD78 alpha is a single-copy gene, the human CCL3L1/LD78 beta gene copy number varies within the population. Human CCL3L1/LD78 beta cDNA encodes a 93 aa residue precursor with a 23 aa residue signal peptide that is cleaved to generate a 70 aa mature protein.