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<p>The <span class="inline-formula"><i>α</i>_<i>S</i>2</span>-casein (<span class="inline-formula"><i>α</i>_<i>S</i>2</span>-CN) is a member of the casein family associated with milk traits in ruminants, but so far the buffalo <i>CSN1S2</i> gene has not been well understood. In this work, the polymorphisms of <i>CSN1S2</i> in river and swamp buffalo were detected using direct sequencing of polymerase chain reaction (PCR) products. As a result, 13 single nucleotide polymorphisms (SNPs) were identified in the coding sequence (CDS) of <i>CSN1S2</i> in two types of buffalo, of which eight SNPs were non-synonymous. The amino acid changes caused by c.580T<span class="inline-formula">&gt;</span>C and c.642C<span class="inline-formula">&gt;</span>G may affect the function of buffalo <span class="inline-formula"><i>α</i>_<i>S</i>2</span>-CN. A total of 11 <i>CSN1S2</i> CDS haplotypes were defined, and accordingly 11 variants of buffalo <span class="inline-formula"><i>α</i>_<i>S</i>2</span>-CN were inferred and named. The <i>CSN1S2</i> CDSs of both types of buffalo were 669 nucleotides, which encoded a precursor of 222 amino acids (AAs), and the first 15 AAs constitute a signal peptide. The composition and physicochemical characteristics of two types of buffalo <span class="inline-formula"><i>α</i>_<i>S</i>2</span>-CNs were similar but slightly different from those of cattle <span class="inline-formula"><i>α</i>_<i>S</i>2</span>-CN. The <span class="inline-formula"><i>α</i>_<i>S</i>2</span>-CN mature peptides of buffalo and the species of <i>Bos</i> genus contained a casein domain, and their secondary structures were highly consistent, indicating that they are functionally similar. The results here provide initial insights into the variation, characteristics and biological function of buffalo <i>CSN1S2</i>.</p>