Background/Objectives: Phelan–McDermid syndrome (PMS), caused by either chromosome 22q13.3 deletions or pathogenic/likely pathogenic variants in the SHANK3 gene, is a rare neurodevelopmental disorder. Behavioral issues greatly impair the quality of life for affected individuals and their families. This genotype–phenotype study intended to further characterize key behavioral features and their genetic and metabolic correlates in PMS. Methods: We conducted a cross-sectional analysis of data on 56 individuals with PMS. Autistic and related behaviors were assessed with the Autism Diagnosis Interview—Revised (ADI-R) and adaptive behavior skills were assessed with the Vineland Adaptive Behavior Scales-Third Edition (Vineland-3), both covering multiple aspects of communication, socialization and abnormal behaviors. Genetic diagnostic information on deletions or pathogenic variants was supplemented with the sequencing data of nine candidate genes on 22q13.3. Metabolic data were obtained using the Biolog Phenotype Mammalian MicroArray plates (PM-M). Results. Every subject in the cohort presented either prominent autistic behavior or adaptive behavior impairment, 55.4% of them meeting the ASD cutoff in every ADI-R domain and 92.9% scoring in the lowest level of adaptive behavior (range of 20–70). Individuals with SHANK3 variants had lower adaptive behavioral skills than those with 22q13 deletions regardless of deletion size, while genomic parameters were largely unrelated to ADI-R scores. Metabolic profiling identified unique profiles of individuals with PMS compared with controls, while distinct profiles distinguished those who met or did not meet the ADI-R ASD cutoff. Cluster analyses revealed groups of individuals with ASD and other clinical features. Conclusion. This study highlighted the importance of SHANK3 in adaptive behavioral skills and uncovered potential metabolic biomarkers of therapeutic relevance.

Objectives and Background: Individuals with Phelan–McDermid Syndrome (PMS) present with a variety of symptoms, including a breadth of behavioral issues. Clinically assessing behavior in PMS remains challenging due to the overabundance of behavioral assessments and the lack of tools validated explicitly for use in individuals with intellectual disability (ID) and neurodevelopmental disorders. This review sought to suggest which assessment tools would best clinically assess behavior in individuals with PMS. Methods: Validated behavioral assessment tools were identified using a systematic search of the literature, and relevant data for each assessment were extracted. The consensus guidelines for PMS were reviewed. Results: This review identified 131 validated assessment tools that were categorized by the intended age group and into specific behavioral domains: Autism spectrum disorder (ASD) screening, adaptive behavior, restricted and repetitive behaviors, challenging/disruptive behaviors, mental health screening, and other miscellaneous behaviors such as avoidance and impulsivity. Discussion: Based on the 2023 consensus guidelines, suggestions were given on which tools would be best for assessing various symptoms and behaviors in PMS. Choosing the best assessment tools to appraise behavior and related symptoms in individuals with PMS will aid clinicians in decision-making and lead to more personalized treatment plans.