[Watch out for a second train].

We report the case of a man with a primary diagnosis of Waldenström macroglobulinemia. He secondarily presented a diffuse large B cell lymphoma (DLBCL) located in the nasal fossae, which relapsed later in the eye. The diagnosis of these two malignancies is based on a multidisciplinary biological approach using new sensitive and specific techniques. These techniques revealed that the two diseases harbor different B cell clones, indicating a distinct origin. This observation highlights the importance of targeted biological techniques for the diagnosis of these two rare hemopathies. It also shows that it is possible to prove the independent nature of the two tumor clones, thus allowing optimized therapeutic management.