They are homozygous for the dominant SB1 allele at the Sabino 1 locus, which has been mapped to KIT. Sabino-white was one of the earliest dominant white alleles discovered, but was not originally recognized as such, hence the different name.
Individuals with homozygous SB1 at the Sabino 1 locus, linked to the KIT gene, are described as having the Sabino-white phenotype. This allele was originally called something else because it was not recognized as a dominant white allele until later.