FOXO1 and alveolar rhabdomyosarcoma: However, phosphorylation at Ser205 is rapidly lost with a concomitant increase in phosphorylation on Ser209, again mediated by CK2.16, 17 In contrast, we found that PAX3-FOXO1 is phosphorylated on Ser201 and Ser205 during proliferation; this status remains unaltered throughout myogenesis with no increase in phosphorylation at Ser209.15, 16 Therefore, the aberrant phosphorylation of PAX3-FOXO1 may affect normal myogenesis to contribute the development of ARMS.