Trisomy 12 is uncommon in typical chronic lymphocytic leukaemias.

The incidence of trisomy 12 was studied by conventional chromosome analysis in 111 patients referred as B-cell chronic lymphocytic leukaemia (B-CLL). Fluorescent in situ hybridization (FISH) was also applied in 34 of those patients with either a normal karyotype or no analysable mitoses. By karyotyping, trisomy 12 was present in 11.7% (13/111), whereas additional FISH increased the incidence to 14.4% (16/111). When subdividing our cases in either typical CLL (n = 90), fulfilling the FAB classification criteria, or atypical CLL (n = 21), with one or more variations from those criteria, the incidence of +12 by metaphase analysis was 3% and 48%, respectively. Additional FISH increased the incidence to 4% and 57%. The most common aberration in atypical CLL was FMC7 positivity (n = 11), followed by CD5 negativity (n = 8), strong surface immunoglobulin staining (n = 7) and atypical morphology (n = 6). Trisomy 12 could only be demonstrated in a small proportion of neoplastic cells in all positive cases. By FISH and/or karyotyping, all available samples at diagnosis of the disease were positive.