Replacement mutations in B29 genes. Among 20 patients affected with CLL, 10 express replacement mutations in 1 allele of their B29 gene; 1 patient, 5B, expresses replacement mutation on both alleles (1) and (2). Ig domain, Ig-like domain; TM, transmembrane segment; intracytoplasmic, intracytoplasmic domain. Underlined amino acids correspond to the replacement mutations.