Host organisms have evolved multiple mechanisms to defend against a viral infection and likewise viruses have evolved multiple methods to subvert the host's anti-viral immune response. Vaccinia virus (VACV) is known to contain numerous proteins involved in blocking the cellular anti-viral immune response. The VACV E3L protein is important for inhibiting the anti-viral immune response and deletions within this gene lead to a severe attenuation. In particular, VACV containing N-terminal truncations in E3L are attenuated in animal models and fail to replicate in murine JC cells. Monkeypox virus (MPXV) F3L protein is a homologue of the VACV E3L protein, however it is predicted to contain a 37 amino acid N-terminal truncation. Despite containing an N-terminal truncation in the E3L homologue, MPXV is able to inhibit the anti-viral immune response similar to wild-type VACV and able to replicate in JC cells. This suggests that MPXV has evolved another mechanism(s) to counteract host defenses and promote replication in JC cells. MPXV produces less dsRNA than VACV during the course of an infection, which may explain why MPXV posses a phenotype similar to VACV, despite containing a truncated E3L homologue. The development of oncolytic viruses as a therapy for cancer has gained interest in recent years. Oncolytic viruses selectively replicate in and destroy cancerous cells and leave normal cells unharmed. Many tumors possess dysregulated anti-viral signaling pathways, since these pathways can also regulate cell growth. Creating a mutation in the N-terminus of the VACV-E3L protein generates an oncolytic VACV that depends on dysregulated anti-viral signaling pathways for replication allowing for direct targeting of the cancerous cells. VACV-E3Ldel54N selectively replicates in numerous cancer cells lines and not in the normal cell lines. Additionally, VACV-E3Ldel54N is safe and effective in causing tumor regression in a xenograph mouse model. Lastly, VACV-E3Ldel54N was capable of spreading from the treated tumors to the untreated tumors in both a xenograph and syngeneic mouse model. These data suggest that VACV-E3Ldel54N could be an effective oncolytic virus for the treatment of cancer.