DNA sequencing studies indicate that only one of two closely linked human embryonic a-like globin genes, zeta (0, encodes a functional polypeptide. The other is a pseudogene($0 that differs by only 3 bp in the protein coding sequence, one of which converts the codon for amino acid 6 into a chain termination codon. Both {-globin genes differ from all other a-like genes thus far reported in that they contain large introns consisting, in part, of simple repeat sequences. lntron 1 of each gene contains a variation of the repeat sequence ACAGTGGGGAGGGG, while intron 2 contains the repeat sequence CGGGG. Comparison of the human {-and a-globin gene sequences reveals that the embryonic and adult a-like genes began to diverge from each other relatively early in vertebrate evolution (400 million years ago). In contrast, the P-like embryonic globin gene, epsilon (E), is the product of a much more recent evolutionary event (200 million years ago). Thus, even though the temporal and quantitative expression of {-and E-globin genes must be coordinately controlled during development, their evolutionary histories are clearly distinct.