The N-terminus of MMSET regulates its localization pattern. Live cell localization of N- and C-terminally GFP-tagged Exon 4a/MMSET III, and N-terminally GFP-tagged B23 and MMSET III–B23 hybrid constructs, in transiently transfected HeLa cells. The live cell permeable DNA stain, Hoechst, is used to identify the nucleus and nucleoli. The Exon 4a/MMSET III construct shows a nuclear localization with nucleolar exclusion pattern. The B23 construct shows the characteristic nucleolar localization pattern, while the MMSET III–B23 hybrid construct shows the typical MMSET phenotype (nuclear and excluded from nucleoli). Immunoblotting experiments of the Exon 4a/B23 constructs confirmed that only the hybrid GFP–Exon 4a–B23 protein product is produced by this expression vector (not shown).