Common blot problems
Western blot diagnostic case library
Each case starts with a blot pattern, lists checks that help separate likely causes, and suggests a next experiment. The illustrations are teaching examples and are not presented as experiments performed by this site.
How to read these cases
The causes are ordered by which checks are most useful to make first; the order is not a calculated probability. The illustrations are original teaching graphics, not raw experimental data, and they do not replace positive or negative controls.
Case 1
Blank blot with little protein on the membrane
Observation: Target, loading control and positive control are absent. The transfer stain shows little or no protein across all lanes.
Blank-transfer comparison
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Uncropped membrane image
- • Post-transfer gel or total-protein stain
- • Sandwich orientation and electrode record
- • Transfer current/voltage trace
Likely causes to check first
- 1. Reversed sandwich or polarity
- 2. Poor electrical contact or exhausted buffer
- 3. Air gap affecting most of the membrane
Next experiment
Repeat transfer with the antibody and detection conditions unchanged; document sandwich orientation and the total-protein stain.
What this result does not show
A blank antibody image alone cannot distinguish failed transfer from failed detection.
Case 2
Loading control present but a large target is weak
Observation: A routine loading control is clear, but a high-molecular-weight target is absent or much weaker than expected.
Large-target under-transfer
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Validated positive sample
- • Target-size region on the uncropped membrane
- • Transfer stain at high molecular weight
- • Post-transfer gel if available
Likely causes to check first
- 1. Under-transfer of the large target
- 2. Poor extraction of the relevant compartment
- 3. Low biological abundance or wrong collection time
Next experiment
Run a validated positive control and inspect the high-MW transfer result before changing antibody concentration.
What this result does not show
A small loading-control band does not show whether a much larger target transferred efficiently.
Case 3
Small target disappears despite acceptable general transfer
Observation: Most proteins transfer, but the expected low-molecular-weight target is absent or inconsistent.
Small-protein loss during transfer
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Membrane pore size
- • Transfer duration and methanol content
- • Post-transfer gel
- • A validated small-protein positive control
Likely causes to check first
- 1. Blow-through or poor retention
- 2. Target below the cropped region
- 3. Antibody incompatibility or low abundance
Next experiment
Compare a shorter transfer or a 0.2 µm membrane while keeping antibody conditions fixed.
What this result does not show
The absence of a low band does not by itself prove blow-through.
Case 4
Uniform dark background across the membrane
Observation: The whole membrane is gray or dark and useful bands appear only at very short exposure.
Uniform-background exposure comparison
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Exposure series
- • Secondary dilution and species
- • Substrate preparation record
- • Wash volume and duration
Likely causes to check first
- 1. Excess secondary antibody
- 2. Overexposure or contaminated substrate
- 3. Insufficient washing or membrane drying
Next experiment
Keep the primary condition fixed and change either secondary concentration or exposure first.
What this result does not show
The appearance alone does not show that blocking is the main cause.
Case 5
Reproducible extra bands at unexpected molecular weights
Observation: The expected band is present, but one or more additional bands recur across experiments.
Extra-band control comparison
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Knockout/knockdown or known-negative sample
- • Known isoforms and processing
- • Independent antibody against another epitope
- • Sample degradation record
Likely causes to check first
- 1. True isoform, cleavage or post-translational modification
- 2. Primary-antibody cross-reactivity
- 3. Degradation or endogenous immunoglobulin signal
Next experiment
Use a genetic negative control or an independent antibody before labeling the extra band nonspecific.
What this result does not show
Molecular weight alone cannot establish whether a band is biological or technical.
Case 6
Vertical smear and broad lanes after high loading
Observation: Lane boundaries are poor and signal spreads vertically, especially in the most concentrated samples.
Loading-dependent smear progression
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Protein load series
- • Lysate viscosity and clarification notes
- • Salt/detergent composition
- • Freeze–thaw history
Likely causes to check first
- 1. Sample overload
- 2. Viscous or insufficiently clarified lysate
- 3. Degradation or gel incompatibility
Next experiment
Keep antibody conditions unchanged and run a lower loading series first.
What this result does not show
A smear can also be biological for heavily modified proteins and should be checked in the context of the target.
Case 7
Local blank area consistent with a transfer bubble
Observation: A sharply bounded pale region appears at a fixed membrane position and cuts across adjacent lanes.
Transfer-bubble position match
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Ponceau or total-protein image
- • Transfer sandwich photo or assembly record
- • Artifact shape across targets
- • Membrane and filter-paper inspection
Likely causes to check first
- 1. Bubble between gel and membrane
- 2. Fold or damaged filter paper
- 3. Uneven cassette pressure
Next experiment
Reassemble with careful bubble removal and check the whole membrane before antibody incubation.
What this result does not show
Position-dependent loss should not be interpreted as sample-specific biology.
Case 8
Loading control is saturated and distorts normalization
Observation: The loading-control bands look equally black across lanes, while target differences appear large.
Saturation and normalization bias
Original teaching illustration showing the blot pattern beside the checks that help interpret it.
What to check
- • Shorter exposure series
- • Raw pixel saturation check
- • Protein loading dilution series
- • Total-protein normalization image
Likely causes to check first
- 1. Loading control outside the linear range
- 2. Target and control acquired under incompatible exposure conditions
- 3. Unstable loading control under treatment
Next experiment
Repeat acquisition or loading so both target and normalization signals are inside a demonstrated usable range.
What this result does not show
A uniformly black loading-control band is not evidence of equal loading when the signal is saturated.