SPP1 and scleroderma: These claims are supported by the following evidence: (1) OPN-null mice demonstrate reduced collagen deposition versus that in wild-type mice [38]; (2) targeting OPN expression by siRNA in fibroblasts from individuals with scleroderma reduces collagen type I expression in vitro [32]; and (3) delivery of exogenous OPN on dermal fibroblasts has a pro-fibrotic effect [39].