Patients with Fabry disease (FD) and amenable mutations can be treated with the chaperone migalastat to restore endogenous α-galactosidase A (AGAL) activity. Our study demonstrates that repeated AGAL activity measurements in patients’ white blood cells at treatment-naïve baseline and during follow-up is mandatory to assess the in vivo amenability to migalastat. In addition, plasma lyso-globotriaosylceramide (lyso-Gb3) levels seem to be an appropriate tool to measure the biochemical response to migalastat. Patients with very low AGAL activities and increasing lyso-Gb3 levels despite in vitro amenability might not benefit sufficiently from chaperone treatment. (By Dr. rer. nat. Malte Lenders, https://jmg.bmj.com/content/early/2019/04/22/jmedgenet-2019-106005 )