The present invention relates to an inhibitor of Hairy and Enhancer of Split 3 (Hes3) or for use in treating a tumour. The invention further relates to an in-vitro method for diagnosing a tumour, comprising determining the amount of Hes3 protein in a cerebrospinal fluid (CSF)-derived sample obtained from a subject, wherein an increased amount of Hes3 protein is indicative of the presence of a tumour.
