The most common clinical features are:
- short fifth fingers and/or toes
- absent or under-developed nails on the fifth fingers and/or toes
- characteristic facial features (thick, bushy eyebrows, long eyelashes, a wide mouth with thick lips, a flat nasal bridge and a wide nasal tip)
- mild to severe developmental delay (see entry Global Developmental Delay) or learning disability (see entry Learning Disability).
Excess body hair (called hypertrichosis or hirsuitism, depending on the pattern) and sparse scalp hair are also very common.
Expressive speech delay is often particularly affected, although children may also have difficulties with their gross motor skills (e.g. sitting and walking), fine motor skills (e.g. hand use) and with their hearing and vision.
Individuals with Coffin-Siris syndrome often have problems with feeding and growth. They may be more bendy (lax-jointed) or have lower muscle tone than other children. Other problems include spinal curvature, recurrent infections and unusual teeth. Less commonly, heart, gut, brain or kidney problems, a cleft palate, seizures, hernias and tear duct abnormalities may be found. Not all of these features would be expected to be found in one individual.