Comparison of the gel electrophoretic pattern of RT-PCR products from the pretreatment and relapse specimens of case no. 9. PCR amplification used primer pair I as described in the Materials and Methods. M1, Hae III-digested ΦX 174 DNA; M2, 100-bp size standard; H, HL-60 cell RNA; N, NB4 cell RNA; P, pretreatment RNA; R, relapse RNA. PCR band 1, full-length V-form with PML breaksite at nt 1685; band 2, full-length product of comigrating atypical isoforms with breaksites at nts 1576 and 1581; band 3, same as band 1 but lacking exon 5 due to alternative splicing; band 4, same as band 2 but lacking exon 5; band 5, isoform lacking exons 5 and 6 due to alternative splicing.