SOST and Osteopenia: In conclusion, partial IGF-I deficiency alone is responsible for osteopenia, characterized by reduced bone mass determined by densitometry and histology, associated with a reduced expression of several proteins involved in osteoblastic/osteocyte activity (OPG, SOST, CTR, IGFBP-5 and RUNX2) and with an overexpression of proteins promoting osteoclastic actions, providing at least two mechanisms possibly contributing to the observed reduction on bone mass in Hz mice.