Monosomy 1p36 results from a heterozygous deletion of the most distal chromosomal band on the short arm of chromosome 1. Occurring in ∼1 in 5,000 live births, monosomy 1p36 is the most common terminal deletion observed in humans. Monosomy 1p36 is associated with mental retardation, developmental delay, hearing impairment, seizures, growth impairment, hypotonia, and heart defects. The syndrome is also characterized by several distinct dysmorphic features, including large anterior fontanels, microcephaly, brachycephaly, deep-set eyes, flat nose and nasal bridge, and pointed chin. Several genes have been proposed as causative for individual features of the phenotype. In addition, based upon molecular characterization of subjects with monosomy 1p36, several mechanisms for the generation and stabilization of terminal deletions have been proposed. © 2007 Wiley-Liss, Inc.