Genomic organization of the human normal wild-type and the patient A1 and A2 transcripts of the CMP–sialic acid transporter gene. The A1 allele contains a 147T>C substitution and 2 microdeletions (G277 and C281) that generate a frameshift (▨) and the appearance of a premature stop codon at position 327. The A2 allele has a 130-bp deletion covering two thirds of exon 6 and generating also a frameshift (▨) and a premature stop codon at position 684.