Metastasis suppressor protein 1 (MTSS1) promotes the monoubiquitination of the immune checkpoint PD-L1 at K263 mediated by the E3 ligase atrophin-interacting protein 4 (AIP4), which leads to the internalization of PD-L1, endosomal transport, and lysosomal degradation, thus inhibiting the immune escape of lung adenocarcinoma [24]. Here, MTSS1 is linked to lung adenocarcinoma.