The collagen IV molecule is 400 nm long and consists of two alpha 1 (IV) and one alpha 2 (IV) chains. Their genes COL4A1 and COL4A2 form a transcription unit in which they are arranged head-to-head and connected by a short common promoter region. Recently, four additional genes have been discovered. They are similarly arranged in the pairs COL4A3-A4 and COL4A5-A6 and code for additional collagen IV isoforms. in the macromolecular network of collagen IV, which provides the scaffold for basement membranes, the molecules are attached and cross-linked via their like end-regions and they interact laterally with their triple-helical domains in a not yet fully understood manner. Collagen IV is involved in interactions with cells and possesses two specific recognition sites for the integrins alpha 1 beta 1 and alpha 2 beta 1.