Test for the sunshine dilution of the SLC45A2 gene, described in horses that appeared double-diluted (cremello/perlino) but did not carry the classic cream or pearl variants. Complementary to, not a substitute for, clinical examination.

Applicable breed: initially described in a Standardbred × Tennessee Walking Horse cross. Frequencies: limited data; rare variant.

• Marked dilution of the coat, with an appearance similar to that of a double dilute.\n• It does not correspond to the classic cream or pearl genotype.\n• It affects both pigments in combination with other SLC45A2 alleles.

In 2019 Holl and colleagues described, in a Standardbred × Tennessee Walking Horse, a variant in SLC45A2 (C^sun) that explained a double-dilution phenotype not attributable to cream or pearl.

• Interpret the result together with cream, pearl and snowdrop, because they share the gene.\n• Avoid combining SLC45A2 dilution alleles if a markedly diluted phenotype is not sought.\n• Do not assume classic double dilution without genotyping cream and pearl.\n• A negative result does not rule out other dilutions.

The phenotype is explained by the cumulative effect of alleles of the same gene. It is advisable to genotype the SLC45A2 alleles before interpreting a 'false double dilute' horse.

1. Holl 2019, A candidate gene approach identifies variants in SLC45A2 that explain dilute phenotypes, pearl and sunshine (PMID 31006892); OMIA:001344-9796.