Initial karyotypic and FISH studies were complemented by genotypization of CLL patients, leading to the discovery that del11q is monoallelic, often large and includes a minimal deleted region encompassing ataxia telangiectasia mutated (ATM) gene and often also BIRC3, located on the 11q22.2 band, in cis with ATM [7,53]. This evidence concerns the gene BIRC3 and B-cell chronic lymphocytic leukemia.