ATF6 and achromatopsia: For example, mutations in ATF6 were recently identified in some subjects with ACHM who were negative for mutations in the aforementioned phototransduction genes.8, , –11 The ATF6 protein encodes an endoplasmic reticulum (ER) localized transcription factor that helps maintain ER homeostasis, as part of the unfolded protein response (UPR).