Within the 885-base-long incomplete coding sequence for the mouse class I major histocompatibility complex (MHC) antigen H-2Kb heavy chain, we have identified 32 recurring base oligomers (1 decamer, 4 octamers, 9 heptamers, and 18 hexamers), the decamer being T-C-A-C-C-C-T-G-A-G. The compilation of these recurring base oligomers led to the identification of the 45-base-long primordial building block for this class of genes which is: C-C-T-G-C-G-G-A-C-A-A-T-G-T-C-A-C-C-C-T-G-A-G-C-T-G-C-T-G-G-G-C-T-C-T-G-G-G-C-G - T-T-G-A-C. Inasmuch as these base oligomers that recurred within the mouse H-2Kb coding sequence proper also recurred within the adjacent, transcribed noncoding sequence, we conclude that the entire approximately equal to 4,000-base-long germ-line DNA segment containing eight or nine separate exons of the coding sequence for class I MHC antigen heavy chains can be construed as having evolved from tandem repeats of this one 45-base-long primordial building block.