Pre-run decisions
Plan a Western blot before loading the gel
Most expensive failures are already visible in the plan: an unverified positive control, the wrong sample fraction, a target outside the chosen transfer window, or quantification without a linear-range strategy.
Answer six questions first
Target identity
What exact protein, isoform or modification is being detected? What apparent molecular weight is expected in this sample?
Biological context
Why should this cell, tissue or treatment express the target at the collection time point?
Sample compartment
Is the target soluble, membrane-associated, nuclear, cytoskeletal, secreted or enriched in another fraction?
Epitope compatibility
Does reduction, boiling, fixation-like crosslinking or sample chemistry alter the antibody epitope?
Antibody evidence
Has the antibody been validated for western blot in a comparable species and sample? What band pattern does the datasheet show?
Quantification goal
Is the experiment qualitative, comparative or quantitative? What must remain inside the linear range?
Controls are diagnostic tools, not decoration
| Control | What it contains | What it tells you |
|---|---|---|
| Positive sample | Known to contain the target | Proves the complete target-specific chain can work |
| Negative/KO sample | Known absent or reduced target | Tests target-dependent specificity |
| No-primary control | Secondary only | Tests secondary/substrate background, not primary specificity |
| Normalization control | Validated stable loading control or total protein | Supports comparison between lanes |
| Transfer image | Ponceau S or another total-protein stain | Separates transfer and loading problems from antibody problems |
Four linked choices
Gel and expected size
- •Use the gel manufacturer’s separation chart rather than a universal percentage table.
- •Gradient gels are useful when the expected range is broad or controls are far apart in size.
- •Do not place target and loading control so close that bands cannot be resolved or independently quantified.
Membrane and transfer
- •0.2 µm membrane is a practical starting choice for small proteins; larger pore sizes are commonly used for broader routine ranges.
- •Large proteins generally need efficient cooling, sufficient time and transfer conditions that do not over-fix them in the gel.
- •Plan a reversible total-protein stain before antibody incubation so transfer is observable.
Blocking and antibodies
- •Start with the antibody datasheet’s validated diluent and dilution.
- •Milk versus BSA is antibody-dependent; the label “phospho-antibody” alone is not enough to choose.
- •Record catalog number and lot. Different lots may require confirmation even under the same nominal protocol.
Detection and quantification
- •Choose chemiluminescence or fluorescence based on sensitivity, multiplexing and instrument availability.
- •Plan multiple exposures and preserve an unsaturated image.
- •If quantitative comparison matters, pilot sample amount and exposure so target and normalization signal are both in range.
A useful first pilot
Question
Can the validated antibody detect the target in my sample?
Keep fixed
Sample preparation, gel, transfer, secondary, detection and exposure series.
Compare
A validated positive control beside the experimental sample; use supplier-recommended primary conditions.
Decision
If the positive control fails, troubleshoot the method/antibody chain. If it works but the sample fails, investigate biology or extraction.
Next pilot
Only then titrate one variable, such as sample amount, primary dilution or transfer condition.
Stop conditions before loading
Pause when any of these remain unknown: no credible positive control, no expected band position, no antibody validation condition, no transfer-verification plan, or quantitative intent without an unsaturated imaging strategy.
