RT-PCR products, obtained using primers encompassing exons 22 and 23, of RNA extracted from Cos-7 cells transfected with minigene expression constructs containing either the normal or mutant IVS 22 donor splice site. The RT-PCR product derived from RNA of cells expressing the wild-type construct (lane 1) had a predominant band of normal size. The RT-PCR product derived from RNA of cells expressing the IVS 22 mutant construct (lane 2) yielded no visible normally sized band but, instead, displayed 2 prominent higher molecular weight bands, corresponding to fragments with incremental sizes of 36 and 620 bp and a faint band of incremental size of 260 bp. The sequence composition of the different products was verified by sequencing and shown to be concordant with the splicing events illustrated in Figure 1B. M indicates size markers.