We reported on two patients with a de novo marker chromosome of which the origins were successfully identified by FISH using microdissected probes. These probes were established by microdissections of extra chromosomal segments from Carnoy-fixed cells stored at -20 degrees C for several years. Using these probes, we could verify partial 1q32 trisomy in a patient with 17p+ as well as partial 16q2 trisomy in another patient with 4p+.