Genes encoding types I, II and III collagens (fibrillar collagens) contain many discrete-size exons, most of them 54 base pairs (bp) long, in addition to the 45-, 99-, 108- and 162-bp exons1-6. It has been suggested 2,6 that these collagen genes evolved from an ancestral coding unit of 54 bp. Type IV collagen is a specific component of basement membranes and contains two genetically distinct polypeptides, the α1(IV) and α2(IV) chains7-9. It differs from the types I-III collagens in that it contains interruptions in the Gly-X-Y repeat sequence10-12 and does not form ordered fibrillar structures7,13. We have isolated complementary DNA and genomic clones for the mouse α2(IV) collagen chain and here characterize 64-, 123- and 182-bp exons in the Gly-X-Y coding domain of the gene. The data suggest that the α2(IV) collagen gene may have evolved differently from those encoding the fibrillar collagens. © 1985 Nature Publishing Group.