The present invention concerns an in vitro method for diagnosing a myeloid tumour or a lymphoid tumour in a subject, which comprises the step of analyzing a biological sample from said subject by (i) detecting the presence of a mutation in the Ten Eleven Translocation protein family member 2 gene (TET2) coding for the polypeptide having the sequence SEQ ID NO: 2, and/or (ii) analyzing the expression of the TET2 gene; wherein the detection of such a TET2 mutation, of the absence of expression of TET2 or of the expression of a truncated TET2 is indicative of a subject developing or predisposed to develop a myeloid tumour or a lymphoid tumour.