chromosome 1q21.1 duplication syndrome (MONDO_0012915) is a rare condition caused by the presence of an extra copy of a small piece of chromosome 1 in the cells of the body. Signs and symptoms can vary widely among affected individuals. Some individuals have no symptoms, while others may have features such as a large head size (macrocephaly); mild to moderate developmental delay and learning difficulties; autism or autistic-like behavior; heart problems; seizures; and/or and distinctive facial features. This condition can occur sporadically as a de novo mutation (by chance) or can be inherited in an autosomal dominant manner from a parent. Treatment depends on the signs and symptoms present in each individual. Also known as: 1q21.1 microduplication syndrome, 1q21.1 recurrent microduplication (possible susceptibility locus for neurodevelopmental disorders), chromosome 1q21.1 duplication syndrome, isolated cases, dup(1)(q21.1), trisomy 1q21.1.