Free AI-Assisted

Codon Host Fit Checker

Compare multi-host codon compatibility with CAI, 5′ mRNA ΔG, GC profiles, and rare-codon bottlenecks—no account. Built-in AI agent assistant support.

Open tool workspace

Need help with bioinfo analysis? See our bioinformatics services.

Key facts

Key facts about Codon Host Fit Checker
FactValue
Target hostsE. coli K12/BL21(DE3), S. cerevisiae, CHO, Human HEK293, P. pastoris, Insect Sf9, custom Kazusa/CoCUT table
Evaluated metricsComposite Host Fit Score (0–100), CAI, GC%, 5′ mRNA initiation ΔG, Codon Pair Score (CPS), rare/tandem bottlenecks, cis-element/restriction site flags
Sequence limitsUp to 30,000 bp per sequence; automatically truncates non-divisible-by-3 nucleotides with warning
Batch limitUp to 50 FASTA records per batch run
Analysis window50-bp sliding window for GC profile; −30 to +30 nt initiation window for mRNA folding ΔG
Output formatsPlain-text diagnostic summary, multi-host CSV, batch JSON, publication SVG/PNG (300 DPI) sequence tracks
Runs in browserYes — 100% client-side execution; sequence data never uploaded
Account requiredNo
AI assistantBuilt-in; validates sequence inputs, explains host metrics, and interprets diagnostic warnings

What it does

Vendor codon optimizers score one host at a time and hide how CAI, mRNA structure, GC islands, and rare-codon runs interact—so a “high CAI” sequence can still fail in CHO while looking fine for E. coli. Codon Host Fit Checker evaluates one or many DNA sequences against multiple expression hosts in a single workspace with transparent metrics and no server upload.

Paste a coding sequence or multi-FASTA (up to 50 records, 30,000 bp each) and select hosts—E. coli K12, BL21(DE3), S. cerevisiae, CHO, Human HEK293, P. pastoris, or Insect Sf9—or paste a custom Kazusa-style codon table. The Multi-host matrix tab shows composite fit score (0–100), CAI, GC%, 5′ initiation-region mRNA ΔG, codon pair score (CPS), and bottleneck/flag counts side by side with a best-host badge.

The Sequence inspector tab opens an interactive codon track (rare codons and tandem bottlenecks highlighted), GC sliding-window plot with publication SVG/PNG export, initiation folding risk, and flagged cis-elements or restriction sites with synonym swap suggestions. Batch benchmark sorts all FASTA entries; Codon tables lists reference host provenance and edits custom tables.

Why researchers use it

  • Compare host compatibility side by side without submitting multiple single-host web forms
  • Identify 5′ mRNA initiation hairpins that block ribosome binding despite high CAI
  • Flag tandem rare codon clusters that trigger ribosomal stalling and premature termination
  • Screen constructs for cryptic splice sites, polyA signals, and internal terminators
  • Process multi-FASTA batches of up to 50 sequences prior to gene ordering
  • Keep proprietary sequence data private with client-side in-browser calculation

Best for

  • Selecting expression hosts (E. coli, yeast, insect, or mammalian) for novel recombinant proteins
  • Diagnosing low yield or truncated expression in existing recombinant constructs
  • Benchmarking vendor-optimized sequences against wild-type genes across multiple hosts
  • Batch QC screening of multi-FASTA synthetic gene libraries before synthesis
  • Customizing codon usage evaluation for non-model organisms or modified host strains

When to use this vs alternatives

Choose Codon Host Fit Checker when you need multi-host benchmarking, transparent 5′ mRNA folding analysis, and batch sequence screening in one private browser session. Use Codon Code Lens when translating open reading frames, back-translating peptide sequences, or exploring raw genetic code tables. Commercial vendor tools (such as IDT or GenScript) suit automated sequence rewriting when you require black-box gene synthesis ordering rather than multi-metric diagnostic evaluation.

What makes it different

Unlike legacy single-host calculators that report only CAI, Codon Host Fit Checker evaluates multi-factorial fitness—combining CAI (35%), 5′ initiation ΔG (25%), GC profiles (15%), rare/tandem bottlenecks (15%), and cis-element screening (10%). It benchmarks multiple host organisms simultaneously in a transparent, vendor-neutral dashboard without uploading sequences to external servers.

Pair with [Codon Code Lens](/tools/codon-code-lens) for translation and back-translation, or [Instant Restriction Mapper](/tools/instant-restriction-mapper) for full enzyme maps. The built-in assistant and API/MCP run the same handler as the UI.

How to get started

  1. Open the workspace on the Multi-host matrix tab and paste your coding DNA sequence into the input field (or click Load example for a demonstration GFP sequence).
  2. Check your target expression hosts (E. coli K12, BL21, S. cerevisiae, CHO, HEK293, P. pastoris, or Sf9) and review the side-by-side composite fit scores and metric breakdowns.
  3. Click any host column to navigate to the Sequence inspector tab for interactive codon heatmaps, 5′ mRNA ΔG folding plots, and GC sliding-window charts.
  4. For multi-sequence projects, select Batch benchmark, drag and drop a multi-FASTA file (up to 50 records), and click Analyze Fit.
  5. Click Copy Diagnostic Report, Download CSV, or export vector SVG sequence tracks for your lab notebook.

Frequently asked questions

How is the composite host fit score calculated?
The composite Host Fit Score (0–100) starts at 100 and subtracts weighted penalties for sequence hazards: CAI shortfall below 1.0 (35% weight), 5′ initiation region mRNA folding energy ΔG (25% weight), overall GC content outside the optimal 40–60% window (15% weight), rare codons (fk < 10%) and tandem bottlenecks (15% weight), and flagged cis-regulatory or restriction enzyme sites (10% weight). Scores above 80 indicate strong predicted expression compatibility.
What does the 5′ mRNA initiation region ΔG metric measure?
The 5′ mRNA ΔG metric evaluates secondary structure stability in the translation initiation window (−30 to +30 nt relative to the start codon) using nearest-neighbor thermodynamic folding parameters. Stable hairpins (ΔG < -8.0 kcal/mol) obstruct ribosome binding and lower protein yield regardless of high CAI. Values between -4.0 and -8.0 kcal/mol indicate moderate structure, while ΔG > -4.0 kcal/mol represents low structure risk for efficient translation initiation.
How does codon pair score (CPS) differ from individual codon adaptation index (CAI)?
While CAI measures the frequency of individual codons relative to host preferences, Codon Pair Score (CPS) evaluates whether adjacent codon pairs are statistically over- or under-represented in the host genome. Even when individual codons are abundant, certain codon pairs create context-dependent ribosomal delays during translation elongation. CPS averages the log-ratio of observed versus expected codon pairs across your sequence, providing a secondary metric for ribosomal translation efficiency.
Can I upload custom codon usage tables for non-model organisms?
Yes. On the Codon tables tab, you can paste or upload custom codon usage tables in Kazusa, CoCUT, or two-column CSV format. The tool computes relative adaptiveness (wi) values and integrates your custom organism into the Multi-host matrix and Sequence inspector tabs. This allows researchers working with non-model bacteria, engineered chassis strains, or specialized synthetic hosts to perform exact compatibility diagnostics client-side.
Can I use an AI agent or MCP with Codon Host Fit Checker?
Yes. The workspace includes a built-in assistant panel that answers field questions, validates input sequences against schema rules, and interprets diagnostic warnings. External AI agents (such as Cursor or OpenClaw) can execute the exact same multi-host calculation handler via [API & MCP](/tools/developers) using the tool identifier pepkio_codon-host-fit-checker and documented agent skills.

Client source code & registry

Open tool workspaceBack to tools

Last updated . Pepkio builds free lab calculators alongside bioinformatics CRO services.