The tetratricopeptide repeat (TPR) motif is a degenerate, 34 amino acid sequence found in many proteins and acts to mediate protein-protein interactions in various pathways. At the sequence level, there can be up to 16 tandem TPR repeats, each of which has a helix-turn-helix shape that stacks on other TPR repeats to achieve ligand binding specificity. TTC21B (tetratricopeptide repeat domain 21B), also known as THM1, is a 1,316 amino acid protein that contains 19 TPR repeats and belongs to the TTC21 family. Localizing to cytoplasm and cytoskeleton, TTC21B exists as two alternatively spliced isoforms and is thought to negatively regulate Shh signal transduction. TTC21B may also be involved in retrograde intraflagellar transport in cilia, and is encoded by a gene that maps to human chromosome 2q24.3.