Lam SW, Cleven AHG, Kroon HM, et al. - Immunohistochemistry using whole sections was conducted on osteoid osteoma (n = 23), osteoblastoma (n = 22), osteoblastoma-like osteosarcoma (n = 3), reactive (n = 3), and proliferative (n = 11) bone lesions in order to assess the immunohistochemical expression of FOS and FOSB in these tumors compared with other bone tumors, to estimate the impact of decalcification, and to relate immunohistochemical findings with the underlying genetic modification using fluorescence in situ hybridization (FISH). In the short decalcified biopsies, as overexpression is observed in the preponderance, FOS immunohistochemistry could be utilized to detect osteoid osteoma and osteoblastoma, being unique in their mimics. Thus, FOS immunohistochemistry should not be used following long decalcification. Furthermore, the low level of focal expression discovered in other lesions and tissues may lead to difficulties in diagnosis, in which case FISH could be used.