ATF4 and Osteopenia: Mice with osteocytes deficient in β‐catenin exhibited severe osteopenia, bone fragility, and a markedly impaired response to mechanical loading.[48, 49] In addition, ATF4 is also mechanistically regulated for terminal osteoblast differentiation and bone formation.[50, 51] However, the interaction between β‐catenin and ATF4 and the mechanisms involved in the regulation of bone mass under microgravity have not been investigated.