Activity of FOXO1 in nonmalignant and Burkitt lymphoma cells. High phosphoinositide-3-kinase (PI3K) kinase activity in nonmalignant cells leads to FOXO1 phosphorylation (P), followed by its 14-3-3 protein-associated nuclear export and proteasomal degradation, whereas FOXO1 regulates target genes in the absence of PI3K signaling. In Burkitt lymphoma cells, mutations in FOXO1 prevent its export from the nucleus in the presence of constitutive PI3K activity, promoting survival and proliferation.