Type IV collagen is a unique member of the large collagen superfamily which in vertebrates comprises 28 different types. Unlike most collagens, type IV collagen occurs only in the basement membranes (BMs) and comprises up to six genetically distinct α-chains designated α1 (IV) to α6 (IV). Out of many potential combinations, the chains interact and assemble with a remarkable specificity to form only three distinct heterotrimers of α1α1α2, α3α4α5, and α5α5α6 (1). It has been shown that ablation of COL4 α 1/2 results in abnormal BM structure and embryonic lethality at E10.5–E11.5, although BM formation during early development is unaffected (2).