The mapping of a balanced t(12;18)(q21;q21.2) translocation in a Sézary syndrome (SS) case led Karenko et al. to identify NAV3 gene (12q21-22) deletion by interphase fluorescence in situ hybridization (FISH) in 15/21 patients with mycosis fungoides (MF) or SS. To determine whether the NAV3 deletion is the result of a specific gene breakpoint, we used FISH with dual-color split or break-apart bacterial artificial chromosome (BAC) probes covering the NAV3 locus. A total of 31 samples (18 skin, 11 blood, 1 lymph node, and 1 spleen) from 24 patients with advanced MF/SS (18 with large-cell transformation) were studied. Chromosome 12 imbalances were analyzed by comparative genomic hybridization (CGH) array with a 3K BAC probes in 24 samples from 22 patients. Both normal FISH and CGH array patterns were observed in 22 samples from 18 patients. In 6 patients, abnormal patterns were observed with an abnormal number of chromosome 12 set in 5 of them. Chromosome 12 structural abnormalities were seen in four of these six patients. An imbalanced FISH pattern between NAV3 and pericentromeric control probes was seen in three patients in accordance with CGH array data (one with a pericentromeric deletion and two with a large 12q deletion including NAV3). No NAV3 specific breakpoint or partial deletion was detected.