HTRA1 and facioscapulohumeral muscular dystrophy: Intriguingly, the serine protease HTRA1 (aFSHD3 myotubes; Fig. 3), was quantified by two peptides with very different H/L ratios, which likely indicates the presence of two isoforms (as described [36]), among which only one was increased in atrophic FSHD myotubes (Fig. S4).