SLC6A1 and Neurodevelopmental delay: However, even though a plethora of missense variants have been reported in SLC6A1-NDD, it is challenging to interpret the pathogenicity of missense variants in SLC6A1. Thus, reclassification of variants from VUS to likely pathogenic or pathogenic variants largely depends on de novo inheritance, a previous pathogenic report of the same variant or functional testing (Richards et al., 2015).