The distinction between benign leiomyomata (fibroids) and smooth muscle sarcomas in the uterus is clear. 1 The genetic changes in sarcomas are more complex than in fibroids [many references over 20 years, available on request]. Interestingly, the specific genetic aberration detected may vary from cell to cell suggesting a high level of genomic instability. This instability can also be detected by techniques that measure allelic imbalance such as comparative genomic hybridization, and is not present in benign fibroids. 2 Some changes are unique to sarcomas such as a heterozygosity for the long arms of chromosomes 10 and 13 that is found in more than half of sarcomas.