Pdcd4 regulates cell proliferation, tumor promotion, and tumor progression via various pathways. Pdcd4 inhibits cell proliferation or cell cycle progression through repression of Cdk1/Cdk2 via the up-regulation of p21Waf1/Cip1 expression, or inhibition of cyclin D1 expression by AKT attenuation, or inhibition of CAII translation. Pdcd4 regulates tumor promotion mainly through inhibition of AP-1-dependent transcription which is controlled by the JNK signaling pathway. Pdcd4 achieves suppression of tumor progression by inhibiting the expression of transcription repressor Snail. Snail negatively regulates E-cadherin expression resulting in activation of β-catenin-dependent transcription and increased expression of c-Myc and uPAR. Subsequently, c-Myc stimulates MAP4K1 expression and thereby activates both JNK signaling pathway and AP-1-dependent transcription. Pdcd4 may also elevate TIMP2 expression that inhibits tumor invasion. In addition, knockdown of Pdcd4 may increase cell survival by induction of p53 translation in response to UV irradiation or treatment with DNA damage agents.