Puromycin vs Blasticidin: What Each Mammalian Marker Kills

Puromycin versus blasticidin mammalian selectable markers are two kill-and-resistance pairs, not two doses of the same drug. Each pair stops translation by a different ribosomal event and is neutralized by a different cassette. A µg/mL value that cleared one parental line does not travel with the other drug, and a day count copied from a vendor table is not a protocol.

Two Mammalian Kill-and-Resistance Pairs

Both agents are used to keep mammalian cells that carry a matching resistance gene and to kill cells that do not. That shared purpose is why they appear in the same CRISPR-plasmid dropdown. Shared purpose is not shared mechanism. If the cassette on the plasmid is pac, blasticidin will not be inactivated by that protein. If the cassette is bsd, puromycin will not be acetylated away.

The audience is a mammalian CRISPR user choosing a kill cassette, or inheriting a vector and guessing the drug from a familiar concentration. The purpose of the page is to make the pair visible: drug plus cassette plus a local kill curve. It is not a trophy for the faster antibiotic, and it is not a best-marker list.

What Puromycin Kills and What Resists It

InvivoGen’s puromycin page describes an aminonucleoside that mimics the 3′ end of aminoacyl-tRNA. The drug enters the ribosomal A site, is incorporated into the growing chain, and causes premature termination. Untransfected cells die because protein synthesis ends in truncated products. That is a chain-termination kill, not a generic “translation inhibitor” badge.

Resistance is typically pac, puromycin N-acetyltransferase. The enzyme acetylates puromycin so it can no longer act as that tRNA analog. Seeing pac on a map tells you which drug the cassette is built to neutralize. It does not tell you how many micrograms your HEK or Jurkat parental line needs, and it does not authorize a swap to blasticidin.

What Blasticidin Kills and What Resists It

InvivoGen’s blasticidin page describes a different ribosomal event. Blasticidin S blocks peptide-bond formation on the ribosome. The growing chain is not terminated by a puromycin-like analog; peptide transfer itself is inhibited. Cells without a resistance gene still die from failed translation, but the chemistry that saved them is not acetylation of puromycin.

Resistance is typically bsd or bsr, a blasticidin S deaminase. That protein inactivates blasticidin. It is a different ORF from pac. Dual selection works only when the two cassettes and the two drugs are both present and both active. One cassette does not cover both kills. Do not treat “mammalian marker” as a single interchangeable part number.

Kill Curves Are Local, Not Swapped Doses

Horizon’s kill-curve protocol is the operational source to keep. Typical starting ranges in that document are 0.5–10 µg/mL for puromycin and 1–20 µg/mL for blasticidin. Those are ranges to test, not doses to copy. The concentration you use is the lowest one that kills 100% of the untransfected parental cells inside that agent’s observation window. Confluent, non-dividing cultures can look falsely resistant. The parental line in your medium is the only legal control.

Published day counts disagree, and that disagreement is the information gain. Horizon’s protocol watches puromycin over a shorter window than blasticidin (days versus a longer span in the same document). Vendor chooser pages quote other clocks — some list blasticidin as several days, others as more than a week. Do not fuse those clocks into one official number. Report the source if you cite a window, then watch your own plates.

Swapping 2 µg/mL puromycin for 2 µg/mL blasticidin because “both are strong” ignores the different typical bands and the different cassettes. Swapping day 3 of puromycin for day 3 of blasticidin ignores the fact that one kill may not have finished. The cassette names the drug. The kill curve names the dose and the wait.

This Is Not Amp Versus Kan

Bacterial plate markers fail in different ways — secreted beta-lactamase versus a more stable aminoglycoside. That pair lives on the ampicillin versus kanamycin page. Satellite colonies on amp agar are not a model for mammalian kill kinetics. Do not import plate-halo logic into a T25 flask.

Choosing among several mammalian CRISPR markers — puromycin, blasticidin, hygromycin, G418 — as a job-fit decision is a later selection page in this series. This page only defines two pairs. After the pair is named, a map such as ZettaGene or a CRISPR design surface such as ZettaCRISPR is one place to see whether the cassette is pac or bsd. Seeing the ORF is not a kill curve. The takeaway is the refusal: kinetics and cassettes are not interchangeable.

Frequently Asked Questions

Can I swap puromycin for blasticidin at the same µg/mL?

No. The drugs, cassettes, and typical concentration bands differ. Run a kill curve for the drug you will actually use.

Do I still need a kill curve if the vector already has pac or bsd?

Yes. The cassette names the drug the cell can inactivate. The µg/mL that clears the parental line is still local.