Fluorescent in situ hybridization (FISH), targeting specific genes/loci frequently affected in CLL patients, has established a standard for stratifying five CLL prognostic groups: del(11q)/ATM, trisomy 12, del(13q) as a sole aberration, del(17p)/TP53, and normal CLL FISH panel results. The gene discussed is ATM; the disease is B-cell chronic lymphocytic leukemia.