Activating transcription factor 4 (ATF4) also known as cAMP-response element-binding protein 2 (CREB2) is a substrate of RSK2 and a basic leucine-zipper transcription factor (PMIDs: 16000305 17485283).ATF4 its a transcription factor that controls the transcriptional activity of mature osteoblasts.ATF4 is particularly critical for their timely onset and terminal differentiation as well as expression of Bsp and osteocalcin.Knockout animals displayed reduction or delay in bone mineralization and have severely reduced bone volume.ATF4 is also part of the PERK-eIF2α-ATF4-CHOP apoptosis pathway which is activated by ER stress and it likely plays a role related to tumor cell survival (PMIDs: 18083928 16000305 30134550).