Automated DNA sequence analysis of PCR products from four APL cases with single nucleotide changes. The upper panels show bi-allelic sequencing results for the pretreatment samples, which contain normal signals as indicated by the arrows in (A), (B), and (C), whereas (D) demonstrates a heterozygous pattern. The middle panels show heterozygous bi-allelic signals for the corresponding relapse samples. The lower panels show the results of nested, PML-RAR allele-specific amplification of PCR products from the relapse cases. (A) Case no. 9, T→C (Met→Thr). (B) Case no. 17, C→T (Arg→Trp). (C) Case no. 14, C→G (Leu→Val). (D) Case no. 2, C→T in 3′-UT region (illustrated from antisense strand sequence analysis, ie, shown as G→A; in the upper panel the heterozygotic mutant A appears on the shoulder of the normal G).