The BCL2 gene is located on chromosome 18q21.33. The BCL2 Break Apart FISH probe is designed to label both ends of the BCL2 gene, respectively. An 800 Kb orange probe is designed at the 3′ end of the BCL2 gene, and an 800 Kb green probe is designed at the 5′ end of the BCL2 gene.

BCL2 gene rearrangements occur in 50% of follicular lymphomas, 23.3% of B-cell lymphomas, and approximately 15% of diffuse large B-cell lymphomas. This translocation results in overexpression of the anti-apoptotic BCL2 protein, which likely constitutes the primary event in malignant transformation.