Mares' milk has a composition very different from that of cows' milk. It is much more similar to human milk, in particular in its casein fraction. This study reports on the sequence of a 994 bp amplified fragment corresponding to a horse αS1-Casein (αS1-Cn) cDNA and its comparison with its caprine, pig, rabbit and human counterparts. The alignment of these sequences revealed a specific pattern of splicing for this horse primary transcript. As in humans, exons 3', 6' and 13' are present whereas exons 5, 13 and 14 are absent in this equine mRNA sequence. BAC clones, screened from a horse BAC library, containing the αS1-Cn gene allowed the mapping of its locus by FISH on equine chromosome 3q22.2-q22.3 which is in agreement with the Zoo-FISH results. Genomic analysis of the αS1-Cn gene showed that the region from the second exon to the last exon is scattered within a nucleotide stretch nearly 15-kb in length which is quite similar in size to its ruminant and rabbit counterparts. The region between αS1- and β-Cn genes, suspected to contain cis-acting elements involved in the expression of all clustered casein genes, is similar in size (ca. 15-kb) to the caprine and mouse intergenic region.

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