Two OB key ancestor bulls born in 1967 and 1974 (ENA SRA sample accession numbers SAMEA4827662 and SAMEA4827664) were heterozygous carriers of a single base pair deletion (BTA24:g.21222030delC) in the MOCOS gene (OMIA 001819–9913) that causes xanthinuria in the homozygous state in Tyrolean grey cattle [19]. Here, MOCOS is linked to xanthinuria.