COL1A1 and osteogenesis imperfecta: Collagen transcripts were variable in mutant cells, with decreased COL1A1 transcripts detected in fibroblasts of OI proband 1/I (p.N459S), IFAP (p.R429H) and KFSD (p.N508S) fibroblasts, but not in OI fibroblasts or osteoblasts with the S2P p.L505F mutation (Fig. 5a).