MONDO_0017334 (12q15q21.1 microdeletion syndrome) is a rare chromosomal anomaly syndrome resulting from a partial deletion of the long arm of chromosome 12, with a highly variable phenotype, typically characterized by developmental delay, learning disability, intra-uterine and postnatal growth retardation, and mild facial dysmorphism that changes with age. Nasal speech and hypothyroidism are also associated. Also known as: Del(12)(q15)(q21.1), deletion 12q15q21.1, monosomy 12q15q21.1.