Phelan-McDermid syndrome is a rare genetic condition usually caused by a deletion or structural change at the end of chromosome 22, in a region called 22q13. Symptoms and severity can vary widely among people with Phelan-McDermid syndrome. For individuals with a chromosome 22 deletion, the size of the deletion may help explain some of these clinical differences. In general, larger 22q13 deletions are associated with more significant developmental delays and medical concerns. A recent study in 63 people with Phelan-McDermid syndrome further explored these genotype-phenotype relationships, helping researchers better understand how chromosome 22 deletion size may influence symptoms such as language development, motor function and development, and social behaviors as well as medical issues. This study also identified several candidate genes that could also be contributing to the symptoms seen in Phelan-McDermid syndrome.