PSME4, also known as proteasome activator PA200, is a regulatory subunit of the proteasome that plays a role in modulating proteasome activity and antigen presentation, particularly in the context of cancer immunology and immune responses. Recent research has highlighted the significance of PSME4 in non-small cell lung cancer (NSCLC). It has been found that PSME4 can modulate the diversity of antigens presented to the immune system, thereby potentially abrogating antitumor immunity in NSCLC. In the context of hepatocellular carcinoma (HCC), PSME4 has been shown to activate mTOR signaling, which promotes the malignant progression of the disease. High expression of PSME4 in HCC tissues correlates with poor prognosis, and PSME4 may promote HCC cell proliferation via the mTOR signaling pathway. PSME4 is also involved in the regulation of proteasome activity, which is essential for maintaining protein homeostasis. It has been identified as a potential therapeutic target in various human diseases, including cancer, due to its role in the degradation of specific nuclear substrates and its involvement in DNA repair mechanisms. Furthermore, PSME4 has been implicated in the activation of the 20S proteasome, which is crucial for the processing of proteins and the generation of peptides for presentation by MHC class I molecules. This function makes PSME4 an important player in immune responses and the immune system's ability to recognize and target cancer cells.