Expression of MMSET mRNA in myeloma cell lines and normal tissues. (A) A Northern blot containing 2 μg of poly(A)⁺ RNA from each of several normal tissues was assessed for MMSET expression hybridizing with an exon 3 probe. The two major bands of 7.7 and 5.2 kb reflect the two polyadenylation signals in exon 24; the lower bands (4.0, 3.1, and 2.8 kb) correspond to the alternative spliced form of MMSET type I containing exon 11, with polyadenylation at different sites. (B) Northern blot analysis of 15 μg of total RNA from 14 myeloma cell lines hybridized with a 3′ exon 24 MMSET probe. The arrow shows the 7.7-kb band, corresponding to the type II MMSET mRNA, that is polyadenylated at nt 7395 of exon 24. The position of 5.0- and 2.0-kb ribosomal RNAs is indicated. The lower panel shows the ethidium bromide staining of the blotted RNAs. (C) A Northern blot containing 2 μg of oligo-dT selected total RNA from MM cell lines with (JIM3 and UTMC2) and without (KMM1) a t(4;14) translocation was repeatedly hybridized with MMSET probes covering different exons. The size of the bands referred in the text is indicated. All three MMSET probes detected the 7.7-kb band (MMSET type II) that, in UTMC2 but not KMM1, cohybridize to the Iμ probe, consistent with the presence of a t(4;14) translocation. Exon 6-10 and exon 19-23, but not 3′ exon 24 probes, also hybridize to the 5.2-kb type II transcript. In addition, exon 6-10 probes detected an 8.8-kb and lower 4.0- and 3.1-kb bands, corresponding to MMSET type I transcript. In JIM3, the size of the bands detected by the exon 6-10 probes is about 600 bp smaller than in the other lines, because the translocation breakpoint is between exon 3 and 4. To obtain comparable signals, the KMM1 blot has been exposed approximately three times longer.