TTF1 and neoplasm: Conversely, when an intracerebral tumour is detected with small cell phenotype, TTF-1 positivity and absence of GFAP staining in the tumour cells, additional testing for high-molecular-weight cytokeratin, Olig2 expression, mutant IDH1 (R132H) and potentially also IDH-sequencing can prevent identifying the lesion incorrectly as a metastasis instead of ASTRO PNC.