Splice mutation variants expressed in a heterozygous CACT individual. The insertion CACT detected in the intron 6 of the CMP–sialic acid transporter is located at 158 bp upstream from the splice acceptor site (•). This insertion creates a new U2 snRNA site that is in competition with the putative normal U2 snRNA site (○) located 27 bp upstream from the splice acceptor site. Five different transcript variants were detected among the 70 sequenced clones: wild-type gene (47%), partial skipping of exon 6 (29%), full skipping of exon 6 (14%), full skipping of exons 5 and 6 (9%), and partial skipping of exon 6 plus full skipping of exon 3 (1%).