IGF1 and Rett syndrome: Although CSF levels of Insulin-Like Growth Factor 1 (IGF1) have been found to be unchanged in RTT (Riikonen, 2003), both the full IGF1 molecule, and its functionally active cleavage product (1-3)IGF1 have been shown to ameliorate symptoms of the RTT pathophysiology (Tropea et al., 2009; Castro et al., 2014; Khwaja et al., 2014; O’Leary et al., 2018).