Myeloma cell lines carrying t(4;14) overexpress the MMSET protein. (A) The schematic map of the MMSET gene, its wild-type (MMSET-I and MMSET-II) and shorter transcripts resulting from variation in the position of the breakpoint in the t(4;14) translocation. Three transcription start sites (bent arrows) within the first 6 exons of the MMSET gene serve as initiation sites for different transcripts depending on the position of the breakpoint (MB4-1, MB4-2, MB4-3) (modified from Keats et al²³ ). The different transcripts encode a different complement of chromatin-related MMSET domains (PWWP, HMG, PHD, and SET). (B) Immunoblot of nuclear extracts from t(4;14)-positive (lanes 1-3) and t(4;14)-negative (lanes 4-6) myeloma cell lines with a rabbit polyclonal anti-MMSET antibody. The KMS11 line overexpresses the full-length isoforms MMSET-I and MMSET-II (lane 1), whereas the H929 and KMS28PE lines overexpress shorter variants MB4-2 (I and II) and MB4-3 (I and II) originating from 2 alternative start sites (lanes 2 and 3). GAPDH is used as loading control. (C) An immunoblot of KMS11 cell extract with rabbit anti-MMSET antibody was coincubated with GST or the GST-MMSET immunogen to check antibody specificity.