The cyclin D1 transcript is subject to alternative splicing. Alternative splicing is known to occur at the exon4/intron 4 boundary. Effective splicing at this junction results in the well-characterized cyclin D1 product (bottom left), which contains discrete motifs that regulate cell cycle control, subcellular localization, and transcriptional regulation. Failure to splice at the exon4/intron 4 boundary results in the cyclin D1b protein product, which harbors a divergent C-terminus (hatched region). The G/A870 polymorphism (circled) is thought to influence the splicing event.