A well-characterized three-chain peptide [(Col1)2 X T9] from human type III collagen was a rich source of Ehrlich chromogen. The corresponding two-chain peptide [(Col1)2] was not, implying that the Ehrlich chromogen is a trifunctional cross-link. (Col1)2 X T9 also contained pyridinoline, which is not an Ehrlich chromogen. The 7S domain of type IV collagen also contained an Ehrlich chromogen.
The structural genes for alpha 1 and alpha 2 chains of human type IV collagen are divergently encoded on opposite DNA strands and have an overlapping promoter region.