Analysis of RT-PCR-amplifiedSlgene transcripts. (A) Total RNA was isolated from the spleen of a C57BL/6J control mouse (+/+) and a heterozygous KitlSl-20J mouse (KitlSl-20J/+) with a white belly spot and RT-PCR was performed using primers that amplified the Sl gene transcripts. Two bands amplified with RNA isolated from +/+ mice corresponded to the membrane-associated (lane 2, band a) and soluble isoforms (lane 2, band b). An additional band (lane 3, band c) was detected with RNA isolated from heterozygous KitlSl-20J mice. M indicates marker lane. (B) Sequence analysis of the amplified soluble isoform of Sl gene transcript from heterozygous KitlSl-20J mouse. The primary copy of exon 3 is underlined and the tandem in-frame duplicated sequence is denoted in bold. The duplication of exon 3 was also present in the membrane-associated isoform of the mutant KitlSl-20J transcript (sequence not shown here). The exon boundaries are indicated by arrowheads.