How to Interpret Western Blot Results: Band Size, Extra Bands and Controls

To interpret a western blot, compare the band you see with the size you expect for your target, work out which direction any shift goes, and then use controls to decide whether the band is really your protein. A band at the expected size is consistent with your target but does not prove it; a band at an unexpected size has a short list of known biological and technical explanations that you can test in order.

This guide covers reading a finished blot. For the bench steps that produce it, see western blot protocol basics and SDS-PAGE gel electrophoresis. If you have no bands at all, use the no-bands troubleshooting decision tree.

What you need

• The developed blot image with a molecular-weight marker lane visible.
• The calculated (theoretical) molecular weight of your target, and any published information on its isoforms, cleavage products or modifications.
• A loading control or total-protein stain for the same membrane (see western blot loading controls).
• Control lysates where available: a positive control (overexpression lysate, reporter line or purified recombinant protein) and a negative control (knockdown or knockout lysate).

Steps: how to read a western blot

1. Check the lanes before the bands. Confirm the marker lane resolved and the loading control or total-protein signal is present in every lane. Differences in your target band mean little if the loading differs.
2. Estimate the observed size. Read your band against the marker and compare it with the calculated molecular weight of the target.
3. Decide whether the band matches. A band at the expected size is consistent with the target. It becomes stronger evidence only when the controls in step 5 behave as predicted.
4. If the band is off-size, note the direction. Higher or lower than expected points to different explanations, listed in the next section.
5. Test the identity of the band. Use the negative control (the band should disappear or shrink in knockdown or knockout lysate) and the positive control (the band should appear or strengthen).
6. Only then compare lanes. Quantification across lanes depends on normalization; see western blot densitometry in ImageJ.

What can go wrong: why a band is not where you expect

The explanations below draw on manufacturer technical guidance from Proteintech, Cell Signaling Technology and Boster Bio (sources checked 2026-10-29). This is vendor guidance rather than a standard, so treat it as a checklist of hypotheses to test, not a verdict.

Bands lower than expected

• Cleavage of a signal or pro-peptide. Proteintech gives PINK1, whose 65 kDa precursor is cleaved into a 52 kDa mature form, and caspase 3, whose proenzyme runs at 32 kDa (p32).
• Isoforms. Proteintech lists glutaminase isoforms KGA (65 kDa) and GAC (58 kDa), and PARD3 isoforms at about 100, 150 and 180 kDa.
• Proteolytic degradation during lysis, which produces lower-molecular-weight fragments.

Bands higher than expected

• Glycosylation. PD-L1 has a full-length size of 33 kDa but is heavily N-glycosylated, with an apparent molecular weight of 45 to 70 kDa. CD133 runs at 115 to 120 kDa and shifts to 75 to 85 kDa after PNGase F treatment, a useful test because the shift itself is evidence for glycosylation. Proteintech also notes that the decorin precursor runs at 43 to 47 kDa.
• Phosphorylation. One phosphate shifts the band by about 1 kDa, often below what SDS-PAGE can resolve; several sites shift it more.
• Ubiquitination. Ubiquitin is a small protein of about 8.6 kDa, so attachment raises the apparent size and deubiquitinases lower it.
• Complexes that survive SDS and reducing agent. Proteintech cites NQO1 homodimers at 66 to 70 kDa and the MLXIP-MLX heterodimer at 130 kDa.
• Multimers. Boster Bio notes that multimers appear at roughly 2x, 3x and 4x the monomer weight. Cell Signaling Technology describes SOAT1 bands at 90, 135 and 180 kDa against a 45 kDa monomer as oligomers, seen with overexpressed or immunoprecipitated samples. Its lysis buffer contains no SDS or DTT, which the authors speculate is responsible, and once formed the oligomers cannot be broken up by SDS or DTT treatment; endogenous lines lysed in an SDS and DTT buffer do not show them.

More than one band

Boster Bio lists splice variants, degradation, overloading and too high an antibody concentration as sources of non-specific or extra bands. Proteintech also names antibody cross-reactivity and recommends positive and negative controls such as knockdown or knockout lysates to sort it out. Boster Bio gives p53 as an example of a target whose observed size, about 53 kDa, differs from the theoretical size, about 43 kDa.

Worked reading order

Observation	First explanations to test	Test to run
Band much higher than calculated size	Glycosylation, ubiquitination, multimer or complex	Deglycosylation shift (CD133 with PNGase F); check for 2x, 3x, 4x monomer sizes; check lysis conditions
Band lower than calculated size	Cleavage to a mature form, isoform, degradation	Compare with published mature size (PINK1 52 kDa); use fresh lysate
Several bands	Isoforms, degradation, overloading, antibody concentration, cross-reactivity	Knockdown or knockout lysate: the specific band should disappear
Band at expected size	Likely the target, not yet proven	Positive and negative controls

What a western blot can and cannot tell you

• It can show the apparent size and relative abundance of a protein the antibody recognises, when loading is controlled.
• It cannot show a phosphorylation change smaller than the gel can resolve: a single phosphate shifts the band by only about 1 kDa.
• It cannot by itself prove a band is the target. A band at the expected size is consistent with the target; knockdown or knockout lysate is what tests the claim.
• Quantitative statements need normalization to a loading control or total protein, as covered in the loading control guide. For reagent logic behind a knockdown control, see shRNA knockdown design and validation.

When to get professional help

If a band cannot be explained after the checks above, ask a colleague experienced with the target or contact the antibody supplier’s technical support. Antibody vendors publish technical notes on unexpected band sizes, and the antibody’s datasheet may list the expected and observed sizes. For a related reagent decision, see polyclonal vs monoclonal antibodies.

FAQ

What does it mean if my band is a different size than expected?

It means the protein migrates differently from its calculated weight. Modifications such as glycosylation, ubiquitination or phosphorylation, cleavage, isoforms and complexes that survive SDS can all do this, so test them in turn rather than assuming the antibody is wrong.

How do I know which band is my protein?

Use controls. The specific band should be lost or reduced in knockdown or knockout lysate and present or stronger in a positive control such as an overexpression lysate, reporter line or purified recombinant protein.

Why do I see bands at two or three times the expected size?

These can be multimers, which Boster Bio says appear at about 2x, 3x and 4x the monomer weight. Cell Signaling Technology reports SOAT1 oligomers of this kind that SDS and DTT cannot break up once formed.

Sources

• Proteintech, “Why does the observed protein MW differ from the calculated one?” (western blot troubleshooting), https://ptglab.com/support/western-blot-protocol/western-blot-troubleshooting-why-does-the-observed-protein-molecular-weight-mw-differ-from-the-calculated-one, checked 2026-10-29.
• Cell Signaling Technology, technical support note on multiple high-molecular-weight bands with SOAT1 (E5O4K) Rabbit mAb #84476, https://www.cellsignal.com/learn-and-support/technical-support/why-do-i-observe-multiple-high-molecular-weight-bands-on-my-western-blot-when-using-soat1-e5o4k-rabbit-mab-84476/000001685, checked 2026-10-29.
• Boster Bio, “Common troubleshooting tips for western blot results, part 1,” https://bosterbio.com/blog/post/common-troubleshooting-tips-for-western-blot-results-part-1, checked 2026-10-29.