In OI skin fibroblasts, the amount of intracellular and secreted type I procollagen was reduced by about half compared to normal cells, while the exposure of OI cells to RA at concentrations of 0.1–100 μM, to RE at concentrations of 0.1–100 μg/mL, and to LBE at concentrations of 0.1–100 μg/mL resulted in a significant increase in the secreted procollagen at all concentrations (Figure 5A). The gene discussed is COL1A2; the disease is osteogenesis imperfecta.