COL6A2 and Ullrich congenital muscular dystrophy: Considering the high frequency of mutations affecting the COL6A2 C-domain, the rarity of UCMD/BM mutations involving the COL6A1 C-terminal region is intriguing, especially in view of the structural and evolutionary similarities between COL6A1 and COL6A2 genes, and thus far remains unexplained [22].