They are uncommon entities and comprise 5% of all cases of small round cell tumors.17 But new research suggest that while the degree of neuronal differentiation used to be applied to distinguish between classical ES and PNET, molecular biology studies have now shown that all these tumors share a common gene rearrangement involving the EWS gene on chromosome 22, so that this distinction is now obsolete. This evidence concerns the gene EWSR1 and primitive neuroectodermal tumor.