However, derivation of SETTLE from the thymus has been debated by some authors [190–193], based on [1] negative immunostaining for CD20, a B-cell marker expressed in epithelial cells in a proportion of spindle cell thymomas; [2] negative immunostaining for CD5, a leukocyte marker expressed by several thymic carcinomas; [3] the recent demonstration of RAS gene mutations in thyroid SETTLEs, not been reported in thymic epithelial tumors; and [4] absence in SETTLE of terminal deoxynucleotidyl transferase (Tdt)–positive immature lymphocytes, typically peculiar to thymomas. This evidence concerns the gene CD5 and thymic carcinoma.