ATM and ataxia telangiectasia: Loss‐of‐function mutations in ATM lead to the classic form of AT that presents clinically with progressive ataxia often starting before age 4 years as wells as the hallmark conjunctival telangiectasias, oculomotor apraxia, extrapyramidal movements (eg, chorea, dystonia, myoclonus, or tremor), elevated serum α‐fetoprotein (often >1,000μg/L), immunodeficiency with recurrent infections, and an increased risk of developing malignancy, often leukemia or lymphoma.52