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DAPI in flow cytometry is a DNA stain excited by the violet 405 nm laser. The manufacturer describes it as cell impermeant, so it is used mainly on fixed cells, for example to read DNA content, and it needs a violet (or UV) laser and a matching detector.
Short answer
DAPI binds the AT regions of double-stranded DNA and gains about 20-fold in fluorescence when it does. In flow cytometry the manufacturer lists it as a fixed-cell stain, where it stains DNA in proportion to DNA amount and so reports cell-cycle position. Because the dye is cell impermeant, it is also discussed as a membrane-integrity marker in unfixed samples (see below). Either use depends on having a 405 nm (violet) or UV laser and a matching detector.
Definition and scope
DAPI is a fluorescent nuclear stain. Thermo Fisher describes it as commonly used as a nuclear counterstain in fluorescence microscopy, flow cytometry and chromosome staining, and lists counting cells, apoptosis assays and sorting by DNA content among its applications. This page covers the flow cytometry uses only. For how the instrument itself works, see what a flow cytometer is; for the wider workflow, see flow cytometry principles, panel design and gating.
How DAPI is used in practice
Laser and detector
Thermo Fisher states DAPI is excited by the violet (405 nm) laser line, and its diagram lists a 350 nm excitation maximum and a 470 nm emission maximum. Its cell-cycle assay page lists DAPI among fixed-cell stains with UV excitation and 452 nm emission, and notes that a UV laser is required. The two pages give slightly different emission values, so the detector filter on your instrument, not a single number, decides whether the signal lands in your violet channel.
Dead-cell exclusion
Thermo Fisher describes the dye as cell impermeant, although it will enter live cells when used at higher concentrations. For an impermeant dye in an unfixed sample, the expected consequence is that stained cells are those whose membranes no longer keep the dye out, which is the general logic of dead-cell exclusion. The DAPI product page itself does not describe a viability protocol, and because of the higher-concentration caveat the working concentration should come from the vendor product documentation, not from a general guide.
DNA content and the cell cycle
The Thermo Fisher cell-cycle page explains that a DNA content histogram plots cell count against linear fluorescence, and that because the dyes bind DNA stoichiometrically, cells in G2/M (4N) fluoresce about twice as brightly as cells in G0/G1 (2N). That page lists DAPI under its fixed-cell stains. Fixed cells are also the standard application given on the DAPI product page.
Related stains: DAPI, Hoechst and propidium iodide
The table summarises what Thermo Fisher states for each stain in its DAPI product page and its cell-cycle assay page.
| Stain | What the source states | Listed excitation / emission |
|---|---|---|
| DAPI | Cell impermeant, enters live cells at higher concentrations; listed under fixed-cell stains; about 20-fold fluorescence enhancement on binding AT regions of dsDNA | 405 nm laser line, 350 / 470 nm (product page); UV, 452 nm (cell-cycle page) |
| Hoechst 33342 | Cell-permeant alternative recommended for live cells; listed for live-cell cell-cycle analysis, where UV light may be detrimental to live cells | UV or 405 nm, 461 nm (cell-cycle page) |
| Propidium iodide (PI) | Binds DNA and RNA; the FxCycle PI/RNase product contains RNase to keep PI from binding RNA | Not listed in the Thermo Fisher sources cited here |
Where it matters for labs, clinics and supply decisions
A practical checklist that follows directly from the sources:
- Laser first. DAPI needs a violet 405 nm or UV laser. Confirm your cytometer has one, with a detector configured for blue emission, before DAPI goes into a panel.
- Sample state. Dead-cell exclusion relies on the dye staying out of intact cells; for live-cell DNA staining the manufacturer points to the cell-permeant Hoechst 33342 instead.
- Scale. DNA content histograms use linear fluorescence, so check that the DNA channel is displayed on a linear axis.
- Panel slot. A violet-excited dye occupies a violet detector; plan it alongside your other fluorochromes and your single-stain compensation controls.
- Related reagents. Antibody conjugates in the same panel still need titration; for dye families more broadly see flow cytometry dyes.
Common misunderstandings
“DAPI is a live-cell nuclear stain.” The manufacturer calls it cell impermeant and recommends Hoechst 33342 for live cells. “DAPI and PI behave the same.” PI binds RNA as well as DNA, and the cell-cycle page describes an RNase-containing PI formulation to address that; Thermo Fisher makes no such statement about DAPI, so this guide makes none. “One emission number fits all.” The two pages list 470 nm and 452 nm, so use your instrument filter configuration.
FAQ
Is DAPI cell-permeant?
Thermo Fisher describes it as cell impermeant, although it enters live cells at higher concentrations. That is why it is used as a fixed-cell stain and why, in principle, it can mark cells with compromised membranes.
What laser excites DAPI in flow cytometry?
The violet 405 nm laser line, per Thermo Fisher. Its cell-cycle page also states that a UV laser is required for the DAPI and Hoechst listings.
What is the difference between DAPI and Hoechst?
The sources describe DAPI as cell impermeant and listed under fixed-cell stains, and Hoechst 33342 as the cell-permeant alternative recommended for live cells.
Can DAPI be used on live cells?
Impermeance is what makes dead-cell exclusion possible in principle, but Thermo Fisher notes the dye can enter live cells at higher concentrations. For staining DNA in live cells, the manufacturer recommends Hoechst 33342 instead.
Does DAPI show cell-cycle phases?
Yes in fixed cells: because binding is stoichiometric, G2/M (4N) cells fluoresce about twice as brightly as G0/G1 (2N) cells on a linear histogram.
Sources
- Thermo Fisher Scientific, DAPI stain, checked 2026-10-29.
- Thermo Fisher Scientific, Cell cycle assays for flow cytometry, checked 2026-10-29.
