1q21.1 deletion syndrome, also called 1q21.1 microdeletion syndrome is a rare genetic disorder caused by a missing segment on chromosome 1. It affects about 1 in 3,400 to 1 in 4,000 people in the general population. The syndrome is inherited in an autosomal dominant pattern, but about half of cases arise spontaneously (de novo), and the deletion can be passed down from a parent who may have no or only mild symptoms. The age of onset is highly variable, with some features detectable before birth (such as growth delays or organ anomalies on ultrasound), while others—like developmental delays or behavioural issues—may appear in childhood or later. The phenotype is extremely variable: some people have no symptoms, while others may have mild to moderate intellectual disability, microcephaly (small head), distinctive facial features, heart defects, eye problems, skeletal or genitourinary anomalies, and psychiatric or behavioural conditions such as autism, ADHD, or schizophrenia. Penetrance is incomplete, meaning not everyone with the deletion will show symptoms, and even within families, the severity and features can differ widely.