Claim: There are four FoxO family members in humans,  FOXO1, FOXO4, FOXO6, and FOXO3 (a.ka. Forkhead box O3), which belongs to the O subclass of the forkhead family of transcription factors which are characterized by a distinct fork head DNA-binding domain.
Evidence: FOXO1 (widely expressed: muscle, liver, pancreas), FOXO3 (apoptosis, erythroid, longevity), FOXO4 (widely expressed), FOXO6 (liver, skeletal muscle, brain) Members of class O (FOXO- proteins) regulate metabolism, cellular proliferation, stress tolerance and possibly lifespan. FOX (forkhead box) proteins are a family of transcription factors that play important roles in regulating the expression of genes involved in cell growth, proliferation, differentiation, and longevity. The defining feature of FOX proteins is the forkhead box, a sequence of 80 to 100 amino acids forming a motif that binds to DNA. Forkhead box O3, also known as FOXO3 or FOXO3a, is a human protein encoded by the FOXO3 gene.