Tau Score
Published
Also known as: τ score, tissue specificity score, tau
A tissue specificity metric (0–1000) used by the Human Protein Atlas. Higher values indicate expression restricted to fewer tissues. τ ≥ 100 is considered tissue-enriched.
Source: Human Protein Atlas (proteinatlas.org); Uhlén et al., Science 2015
The tau score (τ) is a metric that quantifies how tissue-specific a protein’s RNA expression is. It was adopted by the Human Protein Atlas as the primary tissue specificity classifier.
Formula
τ = Σ(1 - x̂ᵢ) / (n - 1)
where x̂ᵢ = normalized expression in tissue i (relative to max across all tissues)
and n = number of tissuesA τ of 1.0 means expression in exactly one tissue; τ of 0 means perfectly uniform expression across all tissues. HPA scales this to 0–1000 for readability.
Thresholds (HPA Classification)
| Tau Range | Classification |
|---|---|
| τ ≥ 500 | Tissue enhanced (high specificity) |
| τ ≥ 100 | Tissue enriched |
| τ < 100 | Low specificity / ubiquitous |
Practical Notes
- Tau is calculated from nTPM values across all 50+ HPA tissues
- A high tau does not guarantee high absolute expression — a protein can be specific but lowly expressed
- Tau should be read alongside fold-enrichment and nTPM for a complete picture
- Secreted proteins can show artifactually lower tau due to detection in blood-contaminated tissue samples