A monoallelic loss of RB1 and FOXO1 was identified in some cases of cellular angiofibromas, spindle cell lipomas and myofibroblastomas, all of which are benign stromal tumors.35, 36 Whether OS is another stromal tumor with some cases having loss of RB1 and FOXO1 deserves further study. The gene discussed is FOXO1; the disease is spindle cell lipoma.