Therefore, HS can diverge from typical hand stereotypies to the midline or combine with non-RTT-specific stereotypies, such as head nodding and rocking (GRIA2), tongue-protruding movements (IQSEC2) or undetermined “stereotypic movements” (KCNQ2), underscoring the coexistence of features typical or reminiscent of RSS with presumably non-RTT-related MD, which might in some instances be ASD- or DD-related. Here, GRIA2 is linked to dentin dysplasia.