- The genes COL4A5 and COL4A6, coding for basement membrane collagen chains alpha 5(IV) and alpha 6(IV), are located head-to-head in close proximity on human chromosome Xq22 and COL4A6 is transcribed from two alternative promoters. Sugimoto, M., Oohashi, T., Ninomiya, Y. Proc. Natl. Acad. Sci. U.S.A. (1994)