In this study, we assessed, for the first time, **whether the core of the pathophysiology of primary complex motor stereotypes (p-CMS) lies in a deficit of inhibitory control**. Using the reaching arm version of the stop-signal task, previously employed on several different experimental models by myself (Tourette and Obsessive-compulsive patients, Mancini et al. 2018; Parkinson's patients, Mirabella et al. 2012, 2013, 2017, Mancini et al. 2019; epileptic pharmacoresistant patients, Mattia et al. 2012; monkeys Mirabella et al. 2011, Mattia et al. 2013; and healthy subjects, Mirabella et al., 2008, Federico & Mirabella 2014; Marino et al. 2015), we compared inhibitory control of 20 drug-naïve patients with p-CMS (mean age ±SD: 7.4±1.1) carefully selected (please see Valente et al. 2019; Cardona et al. 2016; Robinson et al. 2016; Robinson et al. 2014) with that of 20 age- and gender-matched typically developing children (7.5±1.2). We found that a specific pattern of impairment of inhibitory control characterizes p-CMS. Reactive inhibition (i.e., the ability to stop a response outright when a stop instruction is presented) is significantly impaired in children with p-CMS, while proactive inhibitory control (i.e., the ability to shape the motor strategy according to the context in which subjects are embedded) is similar to that of the control group. In our opinion, such a deficit might explain two critical features of the p-CMS phenotype, i.e., the impairment in reactive inhibition would allow the initiation of CMS, while the still efficient proactive inhibition would enable children to regain self-control. We believe that these results represent an essential step in the understanding of the neuropsychological mechanisms of this disease, and thus they constitute a prerequisite for developing effective treatments, which are almost entirely lacking at present (Peter et al. 2017). Besides, our findings together with evidence from other studies (Mancini et al. 2018; Hulst et al. 2018; Smith et al. 2017) suggest that mechanisms underlying the inability to control impulses are incredibly heterogeneous, i.e., they cannot be generally ascribed as a generic deficit of inhibition. **References** • Cardona F, Valente F, Miraglia D, D'Ardia C, Baglioni V, Chiarotti F. (2016) Developmental Profile and Diagnoses in Children Presenting with Motor Stereotypies. Front Pediatr. 4:126. • Mancini C, Cardona F, Baglioni V, Panunzi S, Pantano P, Suppa A, Mirabella G. (2018) Inhibition is impaired in children with obsessive-compulsive symptoms but not in those with tics. Mov Disord. 33:950-959 • Mancini C, Modugno N, Santilli M, Pavone L, Grillea G, Morace R and Mirabella G (2019) Unilateral Stimulation of Subthalamic Nucleus Does Not Affect Inhibitory Control. Front. Neurol. 9:1149. • Marino BF*, Mirabella G*, Actis-Grosso R, Bricolo E, Ricciardelli P. 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