**3. AT diagnosis**

A clinical diagnosis of AT can usually be made on the basis of the presence of characteristic neurological and non-neurological clinical signs and specific laboratory findings. Laboratory studies may be helpful in diagnosing AT by finding elevated alpha-fetoprotein (AFP) levels after age 1, spontaneous and X-ray-induced chromosomal breaks and/or rearrangements in cultured lymphoblastoid cell lines, and reduced cell survival after irradiation [133]. Imaging studies show cerebellar atrophy, not always apparent on MRI in young children, which does not necessarily correlate with the clinical picture [127].

The most common immunological abnormalities found in AT are deficiency of serum IgA, IgE, and selective IgG subscales, elevated IgM levels, lymphopenia (affecting mainly T lymphocytes), and reduced diversity of the T-cell receptor (TCR) immune repertoire [51, 71]. It is important to combine the symptoms present with the results of laboratory tests. A definitive diagnosis can be confirmed by the absence or deficiency of ATM kinase activity, measured in a lymphoblastoid cell line derived from the patient's blood or in fibroblasts from a skin biopsy, and finally the detection of pathological mutations in the *ATM* gene. As elevated serum AFP levels are observed in ≥95% of patients with AT and these levels should therefore be assessed in any child >1 year of age with unexplained atactic gait [134, 135]. The cause of elevated


