Types XV and XVIII collagen belong to a unique and novel subclass of the collagen superfamily for which we have proposed the name the MULTIPLEXIN family. Members of this class contain polypeptides with multiple triple-helical domains separated and flanked by non-triple-helical regions. In this paper, we report the isolation of human cDNAs and genomic DNAs encoding the alpha 1(XVIII) collagen chain. Utilizing a genomic clone as probe, we have mapped the COL18A1 gene to chromosome 21q22.3 by fluorescence in situ hybridization. In addition, using an interspecific backcross panel, we have shown that the murine Col18a1 locus is on chromosome 10, close to the loci for Col6a1 and Col6a2.