- The Iroquois homeobox gene, Irx5, is required for retinal cone bipolar cell development. Cheng, C.W., Chow, R.L., Lebel, M., Sakuma, R., Cheung, H.O., Thanabalasingham, V., Zhang, X., Bruneau, B.G., Birch, D.G., Hui, C.C., McInnes, R.R., Cheng, S.H. Dev. Biol. (2005)