Sharp Bimodal Proliferation Readout that Makes the Gate Obvious

August 13, 2026

BlueskyReddit
Bioengineering
Rice University
Postdoctoral Researcher

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Company:

BioLegend

Product Name:

Alexa Fluor® 488 anti-human Ki-67 Antibody, Clone Ki-67

Catalog Number:

350508

Proliferation is a key outcome measure in our work on immune responses and tumor cell behavior in the context of metastasis. Ki-67 gives us a single-timepoint proliferation readout without the labeling step that BrdU or EdU require, which matters when we are working with primary human samples we cannot pre-load.

Experimental Design and Results Summary

Applications

Flow Cytometry

Sample

Human PBMCs and PBMC-derived lymphocytes, stimulated versus resting, plus cultured human cell lines as a positive control. Fixed and permeabilized after surface staining, approximately one to two million cells per condition.

Primary Incubation

5 µL per test (100 tests, 200 µg/mL) in permeabilization buffer, 30 minutes at room temperature protected from light, followed by two washes in permeabilization buffer.

Blocking Agent

Human TruStain FcX on live cells prior to fixation, plus normal serum in the permeabilization buffer during the intracellular incubation.

Secondary Incubation

N/A — directly conjugated primary.

Tertiary Incubation

N/A.

Detection

Flow cytometry, Alexa Fluor 488 excited off the 488 nm blue laser, collected in the ~525 nm channel. Isotype control run in parallel through the identical fixation and permeabilization protocol. Analysis in FlowJo.

Results Summary

This is one of the more satisfying intracellular stains to run because the result is genuinely bimodal — proliferating and non-proliferating cells separate into two distinct populations rather than a continuum, so gate placement is obvious rather than arbitrary. Resting lymphocytes sit tightly in the negative population and stimulated samples produce a clear Ki-67-positive fraction that scales with stimulation. Alexa Fluor 488 is a good match here: it is brighter and considerably more photostable than FITC in the same channel, and the improved signal-to-background is noticeable on the nuclear stain where permeabilization raises baseline. Because Ki-67 is a nuclear antigen, the permeabilization step is what determines success — a surface-only permeabilization protocol will not reach it, and we had to use a nuclear-grade buffer set to get the pattern above.

Additional Notes

Use a permeabilization system designed for nuclear antigens; saponin-based surface-permeabilization buffers do not reliably give access to Ki-67. Run an isotype control through the full identical protocol, because intracellular background is much higher than surface background and an unfixed comparison tells you nothing useful. Ki-67 marks all non-G0 cells rather than actively dividing ones specifically, so interpret it as "has left quiescence" rather than as a division count. Store at 2–8 °C protected from light; do not freeze.

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Summary

The Good

Cleanly bimodal result that makes gating unambiguous, and Alexa Fluor 488 is brighter and more photostable than FITC in the same channel.

The Bad

Requires a proper nuclear permeabilization protocol, and the intracellular workflow adds a couple of hours plus cell losses to every experiment.

The Bottom Line

The best kind of intracellular reagent — the biology gives you a clean two-population answer and the fluorophore is bright enough not to blur it.

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