Members of the ADAM superfamily of metalloprotease genes are involved in a number of biological processes, including fertilization, neurogenesis, muscle development, and the immune response. These proteins have been classified into several groups. The prototypic ADAM family is comprised of a pro-domain, a metalloprotease domain, a disintegrin domain, a cysteine-rich region, a transmembrane domain, and a variable cytoplasmic tail. We recently identified a novel member of this superfamily, ADAMDEC1 (decysin). Due to the partial lack of a disintegrin domain and the total lack of a cysteine-rich domain, this protein has been placed in a novel subclass of the ADAM gene family. We have investigated the gene structure of the human and mouse ADAMDEC1 and have revealed a metalloprotease gene cluster on human Chromosome 8p12 comprising ADAMDEC1, ADAM7, and ADAM28. Our results suggest that ADAMDEC1 has arisen by partial gene duplication from an ancestral gene at this locus and has acquired a novel function. ADAMDEC1 is expressed in the immune system, by dendritic cells and macrophages. The relatedness of ADAMDEC1, ADAM7, and ADAM28 suggests that these proteases share a similar function.