Mutations on the alleles of sickle-cell anemia and human L-myc oncogene were characterized. The sickle cell anemia results from homozygosity of the sickle-cell allele(βS) at the β-globin gene locus. The βS allele is caused by A to T mutation at the second position of the sixth codon of the β chain gene, resulting in the replacement of a glutamic acid by a valine in the expressed protein. To detect the βS allele, we synthesized a 20-oligonucleotide primer. The primer was complementary to the region right next to the point mutation site on the β-globin gene. Therefore, the 3' end of the primer ended right before the point mutation site of the β-globin gene. The primer would be annealed with the DNA template and its 3' end would be extended/labeled with radiolabeled nucleotide using Taq DNA polymerase. By adding only [α-32P]dATP, but omitting all dNTPs, the primer was extended/labeled when (S allele was served as template. By adding [α-32P]dTTP instead of [α-32P]dATP, the primer was extended/labeled when wild type allele (βA) was served as template.