Quantitative fluorescence-based methods have been developed to determine the nuclear concentration of polyamide-fluorescein conjugates in cell culture. Confocal laser scanning microscopy and flow cytometry techniques are utilized to plot calibration curves, from which the nuclear concentration can be interpolated. Upon treatment with polyamide, the concentration in the nucleus of live HeLa cells is calculated to be between 0.1-0.5 μM, which is significantly lower than the 2 μM dosage concentration. In contrast, the observed nuclear concentration in U251 cells is closer to the dosage concentration, indicating a cell line-specific increase in uptake for this class of compounds. Although confocal microscopy and flow cytometry generate disparate values, taken together these experiments suggest that the polyamide concentration inside the cell nucleus is lower than it is outside the cell.Keywords gene regulation; fluorescence; confocal microscopy; flow cytometry Hairpin pyrrole-imidazole polyamides are synthetic ligands that target predetermined DNA sequences with affinities comparable to those of naturally occurring DNA-binding proteins. 1,2 These cell-permeable small molecules have been shown to localize to the nucleus of living cells 3-7 and regulate endogenous gene expression. 8-12 Polyamides selectively bind in the minor groove of DNA according to a set of "pairing rules," where each heterocyclic ring pair targets a specific Watson-Crick base pair. 1,2 The antiparallel pairing of N-methylpyrrole (Py) and N-methylimidazole (Im) aromatic rings targets the C·G base pair, while Im/Py discriminates G·C. 13-16 The Py/Py pair recognizes A·T and T·A, while chlorothiophene can be paired with Py to distinguish T·A at the N-terminus. 17Polyamide-small molecule conjugates have been utilized in a number of applications. 18-21 In particular, with laser microscopy it is possible to visualize the uptake of polyamidefluorophore conjugates as they traffic unaided to the nucleus of live cells. 3-6 This advance has helped to usher in an era of gene regulation. Figure S1), confocal laser scanning microscopy images (Supplementary Figure S2), and experimental details (Supplementary Information).Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. scoring system to rate the extent of nuclear localization in cell culture. 5,6 Upon incubation with polyamide and direct imaging of cells, the fluorescence intensity in the nucleus is compared to that in the medium. Positive scores are assigned when the nuclear staining exceeds that of the medium. The degree of nuclear localization varies across p...