SOX2 and seminoma: In vivo a subpopulation of SOX2‐deficient cells initiated differentiation into a cell type resembling a mixed non‐seminoma indicated by up‐regulation of germ layer differentiation markers HAND1, PAX6, CDX1 and FOXA2, the trophoblast stem cell / choriocarcinoma marker EOMES and the yolk sac tumour marker AFP42.