Gel blot hybridization studies on DNA from individuals heterozygous for different types of hereditary persistence of fetal hemoglobin (HPFH) have shown a good correlation in gene structures with those deduced from data obtained in studies of the hemoglobins found in members of the families. The recent data have also shown that in certain individuals additional strong hybridizing bands are present. The origin of these bands is as present unresolved, but it may be that they represent additional genes.