Hamster CMP–sialic acid transporter transcripts of CHO and Lec2 cells. The heterozygous exon 2 deletion is present in both CHO and Lec2 cell lines, whereas the homozygous deletion of exon 6 is present only in the mutated Lec2 cell line. This deletion is generated by the G>A substitution in the donor splice site of intron 6 (IVS6 + 1G>A). aa, amino acid.