Sequence of a fragment of the CMP–sialic acid transporter gene of the maternal genomic DNA amplified with the underlined primers (Table 1 ). This fragment contains the 3′ terminus of intron 5, the exon 6, the intron 6, and the beginning of exon 7 (exons in capital letters and introns in lowercase letters). The sequence of the intron 6 revealed the homozygous insertion of 4 bases CACT (bold capitals) inducing a new snRNA U2 site (ACTcaat, double underlined) in all the clones of the maternal genomic DNA. The original putative snRNA U2 site at 27 base pairs downstream from the acceptor splice site (gagtgat) is also double underlined. The sequence of the eukaryote consensus snRNA U2 site is YNYYRAY.^23,24