Activating transcription factor 4 (tax-responsive enhancer element B67), also known as ATF4, is a protein that in humans is encoded by the ATF4 gene. ATF4 transcription factor is also known to play role in osteoblast differentiation along with RUNX2 and osterix. Terminal osteoblast differentiation, represented by matrix mineralization, is significantly inhibited by the inactivation of JNK. JNK inactivation downregulates expression of ATF-4 and, subsequently, matrix mineralization. IMPACT protein regulates ATF4 in C. elegans to promote lifespan. ATF4 is also involved in the cannabinoid Δ9-tetrahydrocannabinol–induced apoptosis in cancer cells, by the proapoptotic role of the stress protein p8 via its upregulation of the endoplasmic reticulum stress-related genes ATF4, CHOP, and TRB3.