Chromosome 1p deletion is a chromosome abnormality that occurs when there is a missing copy of the genetic material located on the short arm (p) of chromosome 1. The severity of the condition and the signs and symptoms depend on the size and location of the deletion and which genes are involved. Features that often occur in people with chromosome 1p deletion include developmental delay, intellectual disability, behavioral problems, and distinctive facial features. Most cases are not inherited, but people can pass the deletion on to their children. Treatment is based on the signs and symptoms present in each person. The disease is Monarch Disease Ontology term MONDO_0016883 (partial deletion of the short arm of chromosome 1). Also known as: partial deletion of chromosome 1p, partial deletion of the short arm of chromosome type 1, partial monosomy of chromosome 1p, partial monosomy of the short arm of chromosome 1.