BTK and B cell deficiency: The patient’s clinical history began with recurrent infections in early infancy, and although immunological testing at 3 months of age revealed severe hypogammaglobulinemia and B-cell deficiency, further diagnostic work-up was lacking at that time, and the pathogenic BTK variant was not identified by genetic testing until 5 years of age, which delayed the definitive diagnosis of XLA and likely contributed to the progression of the neurodegeneration.