NOTCH1 and mantle cell lymphoma: By contrast, while our identification of a novel activating NOTCH1 deletion in the mantle cell lymphoma cell line REC-1 and prior identification of NOTCH1 mutations in primary cases of mantle cell lymphoma [7] clearly indicate an oncogenic role for NOTCH1, our failure to detect NOTCH1 activation or NOTCH1 mutations in 53 well characterized mantle cell lymphomas suggests that involvement of NOTCH1 in this disease is rare.