JUN and plasma cell myeloma: It has been reported that PD-L1 expression was regulated by MEK activation in glioma or myeloma.51 The importance of MEK/ERK pathway in regulating apoptosis after MMC treatment has also been previously suggested.54 ERK1/2 and JNK are direct kinases modulating c-JUN activity,35 therefore these kinases might be involved in the modulation of PD-L1 expression in response to MMC.