Many of the genes of simple organisms with small genomes are encoded on opposite DNA strands so that the genes either overlap or one gene is nested within another gene (Normark, S., Bergstr?m, S., Edlund, T., Grundstr?m, T., Jaurin, B., Lindberg, F.P., and Olsson, D. (1983) Annu. Rev. Genet. 17, 499-525; Chen, C., Malone, T., Beckendorf, S.K., and Davis, R.L. Nature (1987) 329, 721-724). In contrast, most of the genes of complex organisms are dispersed in the genome in widely separated locations. Here, we report that the genes for the alpha 1 and alpha 2 chains of human type IV collagen are encoded on opposite DNA strands from loci that are so closely located that they may be separated by as little as 42 base pairs. The results provide the first description of two structural genes from a complex organism that code for two polypeptide chains of the same protein molecule but have overlapping 5'-flanking regions.