The nucleotide sequences of the two 5'-homology blocks of human alpha-globin gene duplication units were determined. The sequence difference between the two blocks is essentially zero in the 5' portions, and increases gradually toward the 3' ends until it reaches a value of 18 percent. This gradient of sequence divergence is similar to the distribution of the frequencies of gene conversion along several loci in Ascobolus and yeast. Hot spots for initiation of gene correction processes appear to exist near the 5' ends of the human alpha-globin duplication units. The data provide the physical evidence for polar gene correction process in a mammalian genome.