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Detailed studies of chromosome rearrangements within solid tumors require karyotype analysis after cell culturing. However, different cell subpopulations with various growth capacities within one tumor may introduce biases in karyotype analysis, known as the in vitro selection. In our laboratory, 22% of karyotypes from breast cancers established after short-term culture were normal. Using interphase fluorescence in situ hybridization (FISH) for the determination of chromosome 1 arm imbalances and flow cytometry measurements of ploidy, we demonstrated that at least 2/3 of these tumors were mainly composed of aneuploid cell populations. Thus, the incidence of normal or balanced karyotypes among breast cancers is probably below 7%. This is the first direct proof for the existence of an in vitro selection within breast cancer cultures, suggesting cautious interpretation of cytogenetic data.