PITX1 and osteoporosis: Given that defective mesenchymal stem or progenitor cell self-renewal was previously reported as age-dependent osteoporosis by Bonyadi et al., (2003) in Sca-1/Ly6A-null mice10, this prompted us to investigate a possible role of Pitx1 in the transcriptional regulation of the Sca-1/Ly6A gene.