A 14-month-old child has had multiple infections since birth, including pneumonia with Pseudomonas aeruginosa, adenovirus, and Aspergillus fumigatus; diarrhea with Isospora belli; otitis media with Haemophilus influenzae; and urinary tract infection with Candida albicans. Laboratory studies show hemoglobin, 13.2 g/dL; hematocrit, 39.7%; platelet count, 239,100/mm3; and WBC count, 3450/mm3 with 85% segmented neutrophils, 6% bands, 2% lymphocytes, and 7% monocytes. Serum immunoglobulin levels are IgG, 118 mg/dL; IgM, 14 mg/dL; and IgA, 23 mg/dL. The child dies of pneumonia. At autopsy, a hypoplastic thymus, small lymph nodes that lack germinal centers, and scant gut- associated lymphoid tissue are seen. Which of the following is the most likely cause of this disease?