The heterogeneous phenotype in the affected Vallhunds could be due to the unusual overexpression of MERTK. Interestingly, a recent study has identified the Tyro3 gene as a genetic modifier of retinopathy in the Mertk knockout mouse, with allelic variation at Tyro3 moderating the clinical manifestation of the disease [54]. Here, TYRO3 is linked to retinal disorder.