# DBT JRF 2022 — Question 125

Exam: DBT JRF 2022 · Section: B · Topic: Bioinformatics › BLAST & E-value Interpretation · Difficulty: Medium

## Question

Which one of the following is comparatively the best E-value to a pairwise sequence match?

## Options

- **A.** 1e-4
- **B.** 10e-3
- **C.** 100e-2
- **D.** 1000e-1

## Answer

Option A

## Explanation

E-value in BLAST: lower is better (more significant): The E-value (Expect value) in BLAST represents the number of random database hits expected with a score equal to or better than the observed score: $E = m \times n \times e^{-\lambda S}$ where m = query length, n = database size, $\lambda$ = Karlin-Altschul parameter, S = alignment score. Interpretation: • E-value of $1\times10^{-4}$ (0.0001): Only 0.0001 random hits expected → HIGHLY SIGNIFICANT — BEST CHOICE • E-value of $10\times10^{-3}$ = 0.01: 0.01 random hits expected • E-value of $100\times10^{-2}$ = 1.0: 1 random hit expected (borderline) • E-value of $1000\times10^{-1}$ = 100: 100 random hits expected → LEAST SIGNIFICANT Rule: LOWER E-value = MORE SIGNIFICANT match. Typical significance thresholds: • E < $10^{-5}$ : Very significant (homologous) • E < 0.01: Likely significant • E > 1: Not significant (chance match) Answer: $1 \times 10^{-4}$ (= 0.0001) is the best (most significant) E-value.

Source: https://mypyqbuddy.com/questions/which-one-of-the-following-is-comparatively-the-6a104cd5920734408bc0c78c