Potential tumor suppressor. Required for death receptor- dependent apoptosis. Mediates activation of STK3/MST2 and STK4/MST1 during Fas-induced apoptosis by preventing their dephosphorylation. When associated with MOAP1, promotes BAX conformational change and translocation to mitochondrial membranes in response to TNF and TNFSF10 stimulation. Isoform A interacts with CDC20, an activator of the anaphase-promoting complex, APC, resulting in the inhibition of APC activity and mitotic progression. Inhibits proliferation by negatively regulating cell cycle progression at the level of G1/S-phase transition by regulating accumulation of cyclin D1 protein. Isoform C has been shown not to perform these roles, no function has been identified for this isoform. Isoform A disrupts interactions among MDM2, DAXX and USP7, thus contributing to the efficient activation of TP53 by promoting MDM2 self-ubiquitination in cell-cycle checkpoint control in response to DNA damage. {ECO:0000269|PubMed:10888881, ECO:0000269|PubMed:11333291, ECO:0000269|PubMed:12024041, ECO:0000269|PubMed:14743218, ECO:0000269|PubMed:15109305, ECO:0000269|PubMed:15949439, ECO:0000269|PubMed:16510573, ECO:0000269|PubMed:18566590, ECO:0000269|PubMed:21199877}. This is the function of RASSF1 (Ras association domain family member 1, Ensembl gene identifier ENSG00000068028).