An example of evolutionary emergence of REPA G tracts between the polypyrimidine tract and 3′AG within the upstream 3′SS of alternative exons of mammalian genes. Shown is an alignment of sequences around the 3′SS of the CaMK1 gene from 22 species. The G tract (black bar in diagram) is at the 3′SS of intron 8 (in front of the exon 9). Retention of both upstream and downstream introns around exon 9 generates a splice variant with an alternative stop codon in the upstream intron (Genbank accession number AB209395). In most other transcripts, both introns are spliced. Arrowhead: intron/exon boundary. It can be seen from the alignment that the G pentamer is present in most mammals. It is absent in wallaby, platypus, birds and the other vertebrates shown here.