Here, in an attempt to prove that GCT of the clivus may be caused by the same genetic alteration of GCT of long bones, we performed the molecular analysis of the GCT-related genes (H3F3A, H3F3B, IDH1 and IDH2) as well as of the ZNF687 gene that we recently identified as gene responsible for giant cell tumour associated with Paget’s disease of bone [3, 6, 28]. The gene discussed is IDH1; the disease is bone Paget disease.