The DNA sequence of the human beta-globin region is strongly biased in favor of long strings of contiguous purine or pyrimidine residues.

The DNA sequence of the human beta-globin region, comprising over 67 kilobase pairs, has been analyzed for the occurrence of strings of contiguous purine or pyrimidine residues. Tracts of 10 or more contiguous residues are found 4 times more frequently than would be expected with a random distribution of bases, so that a long string occurs at an average of every 250 base pairs. A survey of six other human gene sequences, totaling 86 kilobase pairs, shows a remarkably similar result. No such overrepresentation of contiguous purine or pyrimidine residues is found in the bacteriophages lambda or T7.