Collagen composition of extracellular matrix in bones from untreated and treated mutant mice. Type I collagen was extracted from trabecular and cortical bone samples and analyzed by quantitative sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) with fluorescent labeling (A). The fraction of α1(I) chains forming α1(I)-S-S-α1(I) dimers represents the fraction of collagen molecules with 2 mutant chains. This fraction decreased with increasing engraftment of donor (eGFP⁺) cells (r² = 0.58) because they produced only collagen without mutant chains (B). Because host cells are expected to synthesize the same fraction of the dimers in treated and untreated animals, the fraction of collagen made by the donor cells (c_d) can be evaluated as c_d = 1 − d/d₀, where d and d₀ are the fractions of dimers measured in treated and untreated animals, respectively. This fraction increased with the donor cell engraftment (C, r² = 0.78). The engraftment was measured for each animal by FACS in bone marrow (Table 3). The same engraftment in the cortical bone is expected from real-time PCR data (Figure 3 and supplemental Table 1).