Therefore, the presence of two distinct phenotypes—non-FS-STGD1, which is primarily childhood-onset and adulthood-onset STGD1, and FS-STGD1—suggests more than one disease mechanism in ABCA4-associated retinopathy.14 The fact that a different distribution of disease-causing variants exists between these two phenotypes appears to support this hypothesis.14 26. This evidence concerns the gene ABCA4 and retinal disorder.