RB1 and lipoma: On the other hand, SL/PL is consistently positive for CD34 and negative for desmin and S100p [16,23]. Loss of retinoblastoma 1 (Rb1) gene expression is seen in 50-70% of cases of atypical spindle cell/pleomorphic lipoma which is also seen in SL/PL and correlates with the deletion of 13q14 harboring the RB1 locus [23,24]. Well-differentiated liposarcoma (WDLS) is another close differential diagnosis.