TGM2 and atypical Rett syndrome: Cysteamine is an established, commercially available, safely tolerated inhibitor of TG2, initially developed for the treatment of cystinosis but which has since shown promise in the treatment of other diseases such as Huntington’s chorea, Leigh disease, mitochondrial disease, Parkinson’s disease, and Rett syndrome, and it is now being considered for re-evaluation in new disease contexts including cancer, given the growing knowledge of the role of TG2 in the TME [272].