TRIM21, as an E3 ubiquitin ligase, has been reported to be involved in the proliferation and metastasis of many malignant tumors by promoting polyubiquitin and proteasome degradation pathways.[50, 51] In this study, it was found for the first time that HTR1A interacts with TRIM21 and PSMD7, promotes the ubiquitin‐proteasome degradation pathway of TβRII and inhibits downstream signaling pathways. Here, TRIM21 is linked to cancer.