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. This evidence concerns the gene RB1 and spindle cell lipoma.