CFTR and cystic fibrosis: To calculate the trafficking index (TrIdx) of each variant, we transiently transfected HEK293 cells with each of the 64 variants in our library of native mutations responsible for the majority of CF in the clinic, and quantitated the relative abundance of the ER-localized core-glycosylated high mannose containing CFTR band B and the post-Golgi localized band C glycoforms that migrate uniquely on SDS-PAGE in response to trimming and modification by Golgi-localized enzymes (Supplementary Figs. 1–7, 8a upper panel)36.