Partial monosomy 11q, also known as Jacobsen syndrome, is a rare chromosomal disorder in which a portion of chromosome 11 is missing or deleted 11q monosomy. The range and severity of symptoms varies, depending in part on the exact location and size of the missing material. Symptoms commonly associated with partial monosomy 11q include abnormally slow growth before and after birth (prenatal and postnatal growth retardation), and/or moderate to severe delays in the acquisition of skills requiring the coordination of mental and muscular activity (psychomotor retardation). In rare cases, individuals may have normal/borderline intelligence while in most cases children have mild to severe intellectual disability. Characteristic physical abnormalities may include malformations of the head and face (craniofacial), abnormalities of the eyes, hands, feet, bleeding due to platelet abnormalities, and/or defects of the heart that are present at birth (congenital). The exact cause of partial monosomy 11q is not fully understood.