The TP53 gene (tumor protein p53, a.k.a. p53, BCC7, LFS1, TRP53) is located in the chromosomal region 17p13.1 and encodes a 53 kDa transcription factor. TP53 gene deletions have been detected in patients with chronic lymphocytic leukemia (CLL), multiple myeloma (MM), and acute myeloid leukemia (AML). In CLL patients, allelic loss of the short arm of chromosome 17 is associated with treatment failure with alkylating agents and short survival times. Isochromosome 17q is a frequent cytogenetic abnormality seen in hematologic malignancies including blast phase of chronic myelogenous leukemia (CML), AML, Hodgkin and non-Hodgkin lymphomas. In neuroblastoma, gain of the 17q21-qter is associated with stronger tumor progression. Thus, the combined detection of both targets by Fluorescence in situ Hybridization allows for a sensitive determination of isochromosomes and may be a helpful tool for diagnosis and selecting treatment.