# Contamination in sequencing sample

**URL:** <https://scientistsolutions.discourse.group/t/contamination-in-sequencing-sample/572>\
**Category:** DNA\
**Created:** [January 2, 2025, 7:43am UTC](https://scientistsolutions.discourse.group/t/contamination-in-sequencing-sample/572 "2025-01-02T07:43:00Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Danielsuarez1609](https://avatars.discourse-cdn.com/v4/letter/d/e9a140/32.png) [@Danielsuarez1609](https://scientistsolutions.discourse.group/u/Danielsuarez1609)\
**Post date:** [January 2, 2025, 7:43am UTC](https://scientistsolutions.discourse.group/t/contamination-in-sequencing-sample/572/1 "2025-01-02T07:43:00Z")

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Hi,

I always get a comment that my sequencing samples (T vector clone) are contaminated. Can anyone tell me what does that mean?

Thanks in Advance.

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**Author:** ![ARGERINE](https://avatars.discourse-cdn.com/v4/letter/a/8491ac/32.png) [@ARGERINE](https://scientistsolutions.discourse.group/u/ARGERINE)\
**Post date:** [November 28, 2025, 11:06pm UTC](https://scientistsolutions.discourse.group/t/contamination-in-sequencing-sample/572/2 "2025-11-28T23:06:57Z")

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> [@Danielsuarez1609](#):
>
> I always get a comment that my sequencing samples (T vector clone) are contaminated. Can anyone tell me what does that mean?

Daniel Suarez

I was going through some old unanswered posts and found yours. I hope that you’ve already circumvented the issue.

The **most common** issue is that Sanger sequencing requires **one clean plasmid species** + **one primer**. If the sample contains **more than one plasmid sequence** , the chromatogram will show **overlapping peaks** , especially at the beginning. This often happens if you pick a **mixed colony** , have **vector-only plus insert-containing plasmids together** , leftover **PCR product/genomic DNA** , or **contaminated primers**.

**How to fix the issue :** Re-streak and pick a **single isolated colony** , confirm the insert by **colony PCR** , make a **clean miniprep** , and resend for sequencing.