SH3D21 (SH3 domain containing 21) is a protein adapter involved in actin cytoskeleton organization and cell migration. Structurally, it contains an SH3 domain that mediates protein-macromolecule interactions 1. In hepatocellular carcinoma, SH3D21 expression is upregulated and promotes cancer progression by activating the PI3K/AKT/mTOR signaling pathway, enhancing cell proliferation, invasion, and migration while facilitating an immunosuppressive tumor microenvironment 2. SH3D21 also functions as a sensitizer for gemcitabine therapy in pancreatic cancer, suggesting its involvement in drug efficacy pathways 3. In oral cancer and potentially malignant disorders, SH3D21 is among key genes implicated in actin cytoskeleton reorganization associated with increased invasiveness, with ROC curve analysis demonstrating its potential as an early biomarker 1. Differential methylation of SH3D21 genomic regions correlates with outpatient surgeries in carriers of Val122Ile TTR mutation 4. Notably, SH3D21 knockout mice exhibit normal male fertility, indicating the gene is dispensable for reproductive function 5. These findings identify SH3D21 as a cancer-associated adapter protein with potential clinical significance as a diagnostic biomarker and therapeutic target in solid tumors, though its precise mechanistic role in normal physiology remains incompletely characterized.