A subfamily within the forkhead box (FOX) protein family, which is a family of transcription factors that contain a forkhead DNA-binding domain. FOXO proteins regulate apoptosis, cell-cycle progression, oxidative-stress resistance, and tumor suppression. The transcriptional promoter activity of these factors is regulated by post-translational modifications, such as phosphorylation, acetylation, and monoubiquitination. [ ]