COL6A3 and Congenital muscular dystrophy, Ullrich type: Gapmer AONs were designed to selectively bind to the mutant allele to induce target RNA degradation by activating RNase H, an endonuclease that cleaves the RNA strand of a DNA-RNA duplex.17, 18 A skin fibroblast cell line from a UCMD patient carrying a de novo 18-nt genomic deletion in exon 15 of the COL6A3 gene was selected as an in vitro model.