Notably, all three synaptic type IV collagen isoforms, α3- (COL4A3), α4- (COL4A4), and -α5 (COL4A5), are strongly downregulated in FSHD patients compared with controls throughout all datasets (Supplementary Data 1, sheet 2a). Here, COL4A4 is linked to facioscapulohumeral muscular dystrophy.