TTC21A (tetratricopeptide repeat domain 21A) is an intraflagellar transport (IFT)-associated protein essential for male reproductive function. Functionally, TTC21A is required for spermatogenesis and sperm flagellar formation through its role in intraflagellar transport 1. The protein localizes to cilia and participates in intraciliary retrograde transport as part of IFT particle A complexes, containing multiple tetratricopeptide repeat domains crucial for ciliary function 1. Clinically, bi-allelic mutations in TTC21A cause asthenoteratospermia and multiple morphological abnormalities of sperm flagella (MMAF), accounting for approximately 5% of MMAF cases in Han Chinese populations 1. Affected individuals present with reduced sperm motility, abnormal flagellar morphology, and defective head-tail conjunction 1. TTC21A interacts with other ciliary proteins, including CEP78, which regulates its stability and localization at centrioles 2. Male infertility caused by TTC21A mutations is designated Spermatogenic failure 37 3. Beyond reproductive function, emerging evidence suggests TTC21A participates in immune regulation, with increased expression correlating with favorable prognosis in lung adenocarcinoma and head and neck squamous cell carcinoma, associated with enhanced immune cell infiltration 45.