The function of TTC21B (tetratricopeptide repeat domain 21B, Ensembl gene identifier ENSG00000123607) is as follows. Component of the IFT complex A (IFT-A), a complex required for retrograde ciliary transport and entry into cilia of G protein- coupled receptors (GPCRs). Essential for retrograde trafficking of IFT- 1, IFT-B and GPCRs (PubMed:27932497). Negatively modulates the SHH signal transduction (By similarity). {ECO:0000250|UniProtKB:Q0HA38, ECO:0000269|PubMed:27932497}.