The coding sequence for the 97-amino-acid-residue-long immunoglobulin heavy-chain variable (VH) regions of the mouse apparently arose as repeats of the two short building blocks. Three of the recognizable copies of the one 21-base-long prototype sequence A-C-T-G-G-A-T-A-T-G-A-C-C-T-G-G-A-G-T-G-G are invariably found to occupy the fixed positions within the 5' half of each VH coding sequence. Interestingly, the first and third copies specify the relatively invariant regions represented by the 7th to 13th and 41st to 47th amino acid residues (the first and second framework regions), whereas the second copy specifies the first hypervariable region (31st to 35th amino acid residues). These copies maintain at least 57.2% (12 out of 21) base sequence homology to the above-noted prototype building block. Base sequences of the other 14- to 15-base-long prototype building block differ from each other by as much as 60% between individual VHS. Yet one of its copies invariably occupies the terminal region of each VH coding sequence, thus specifying the very invariant third framework region. Other copies occupy unfixed positions in the VH and its attendant hydrophobic leader coding sequence as well as in adjacent noncoding sequences. The homology thus revealed between the VH coding sequence and its adjacent noncoding sequences suggests their concordant evolution.