Start by finding out whether the protein reached the membrane. A Ponceau S stain and a prestained ladder split most failed blots into two halves in a few minutes.
Range: Antibodies3 minute readUpdated
Ponceau S is a reversible protein stain. A few minutes on the membrane after transfer tells you whether protein arrived, whether the lanes loaded evenly and whether the transfer was uniform. It rinses away and does not interfere with antibody detection. A prestained ladder on the membrane gives a second check.
| Ponceau result | Meaning | Look at |
|---|---|---|
| No protein, no ladder on the membrane | Transfer failed | Sandwich orientation, buffer, power |
| Ladder present, no sample protein | Sample problem | Lysis, quantification, loading |
| Protein present, no target band | Detection problem | Antibodies, controls, substrate |
| Patchy or blank areas | Air bubbles in the sandwich | Roll the stack out when assembling |
| What you see | Likely cause | What to try |
|---|---|---|
| Bands below the expected size | Protein degradation | Fresh lysate, protease inhibitors, keep cold |
| Band above the expected size | Glycosylation, phosphorylation, or incomplete reduction | Fresh reducing agent; check the literature for the target |
| Many faint bands | Antibody too concentrated or non-specific | Dilute, block longer, test a knockdown or knockout |
| Bands curve upward at the edges | Gel ran too hot | Lower the voltage or run cold |
If a validated positive control still gives nothing, change the antibody rather than the protocol. Compare clones and hosts in the antibody range, and see monoclonal vs polyclonal for how clonality changes what you see on a blot.
Check the transfer first with Ponceau S. If protein reached the membrane, the usual causes are an antibody not validated for western blot, a mismatched secondary, too little target protein or an exhausted substrate.
Block for longer with fresh milk or BSA, dilute the primary and secondary antibodies, wash three or more times in TBS-T, and never let the membrane dry.
Milk is inexpensive and effective for most targets. Use BSA when detecting phosphoproteins, because milk contains casein, a phosphoprotein that phospho-specific antibodies can bind.
Post-translational modifications such as glycosylation slow migration, degradation produces smaller fragments, and incomplete reduction leaves dimers at about twice the expected size.
For research use only. Protocols here are general outlines: follow your kit manual and your institution’s safety rules. See our editorial policy.
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