Gene expression profiling (GEP) analysis carried out in a proprietary data set of 129 MM, 24 primary plasma cell leukemia (pPCL) patients and 4 normal controls revealed that the Notch transcriptional target HES5 and the Notch ligand JAG1 are overexpressed in the different types of PC dyscrasias compared to normal controls, reaching a higher expression level in pPCLs (Figure 1A). Here, HES5 is linked to plasma cell leukemia.