Fluorescence in situ hybridization to TEL and AML1 in leukemia from a twin patient.TEL deletion is subclonal. Red indicates AML1 probe; green, TEL probe. Metaphase cells have red plus green (=yellow), TEL-AML1 fusion signal. The cell on the left retains normal TEL allele (green signals). The cell on the right has lost normal TEL allele. Patient is twin in triplet set no. 16, data in Wiemels et al.⁶⁸  FISH picture courtesy of Drs C Harrison and G Jalali (University of Southampton, United Kingdom).