ORUCUB is considered an osteoclast-rich undifferentiated UC (a variant of high-grade undifferentiated UC) due to the intimate association of this tumor with high-grade conventional UC and similar IHC profile, including positivity with CKs, EMA, Ki- 67 and p 53 in neoplastic mononuclear cells as well as in the adjacent UC component with non-neoplastic reactive histiocytic lineage of OCGs that are CD-68 positive and Ki-67/p 53 negative. The gene discussed is MKI67; the disease is neoplasm.