BACKGROUND: Clinical findings in Fabry disease have classically been attributed to loss-of- function variants in the GLA gene that result in α-galactosidase A deficiency, intracellular accumulation of globotriaosylceramides and clinical manifestations. However, over time, increasing number of patients have been identified with GLA variants causing either non-classic Fabry disease or having unclear clinical effects. SUMMARY: Searching for additional etiologic and lysosomal storage-independent factors, investigators have recently identified that certain missense GLA variants not only affect enzymatic activity, but also encode for misfolded α-galactosidase A that itself induces chronic endoplasmic reticulum stress and the unfolded protein response. Thus, Fabry disease pathogenesis may be caused by decreased enzymatic activity as well as cellular toxicity from accumulation of the misfolded α-galactosidase A protein, with the contribution of each factor determined by the type of the genetic variant and host factors. KEY MESSAGES: Defective proteostasis and misfolding of certain missense α-galactosidase A variants induce chronic endoplasmic reticulum stress and the unfolded protein response that may contribute to intra-familial and inter-familial variation in disease penetrance and clinical expressivity. Pharmacologic modulation of defective proteostasis may have therapeutic implications in Fabry disease.