C1R and Ehlers-Danlos syndrome, periodontitis type: Both PCPE1 and BMP1, as well as C1s, can bind through their CUB domains to the triple helix of collagen and/or propeptides that can be degraded.31, 32, 33, 34, 35 This suggests that abnormal binding of (mutated) C1r/C1s to connective tissue precursors could be a pathogenetic factor in pEDS.