Introduction of a one-base mismatch into the guide seed region of siRNA significantly influenced allele discrimination and hence allele-specific silencing when tested in targeting the single-nucleotide COL6A1 c.850G>A (p.G284R) mutation,34 and the c.877G>A (p.G293R) mutation,47 responsible for a dominant form of UCMD. The gene discussed is COL6A1; the disease is Ullrich congenital muscular dystrophy.