Similar axon targeting phenotypes have also been reported upon knock-out of L1 family members Nrcam and Chl1 while L1 knockout mice succumb to the much more severe CRASH syndrome with corpus callosum hypoplasia and mental retardation (Heyden et al., 2008; Montag-Sallaz et al., 2002; Demyanenko et al., 2011; Kolata et al., 2008). Here, CHL1 is linked to L1 syndrome.