EN2 and fragile X syndrome: Besides high penetrance monogenic disorders like Fragile X Syndrome (Berry-Kravis et al. 2017), prominent genes that have been identified include Reelin (RELN) (Quattrocchi et al. 2002), MET (Campbell et al. 2008), serotonin transporter (5HTT) and engrailed 2 (EN2) (Bartlett et al. 2005).