fastq
What is a .fastq file?
text/x-fastq
FASTQ pairs each sequence with per-base quality scores. A record is exactly four lines: a `@` header, the sequence, a `+` separator that may repeat the header, and a quality line of the same length whose characters encode Phred scores as ASCII with an offset — 33 in the Sanger/Illumina 1.8+ encoding, 64 in older Illumina data. Misidentifying that offset silently shifts every quality value by 31.
How to use a .fastq file
Use an example .fastq file to test read parsers, quality-trimming tools, and encoding detection, verifying that sequence and quality lengths match, that a `@` at the start of a quality line is not mistaken for a new record, and that the documented offset is applied.
Download example .fastq files
- FASTQ Reads — Phred+33 (Sanger / Illumina 1.8+) (.fastq)Six 60-base reads with Phred+33 quality strings, the modern Sanger encoding used by everything since Illumina 1.8. Quality declines along each read the way real sequencing does, so a mis-decoded offset produces implausible scores rather than plausible ones.
- FASTQ Reads — Phred+64 Legacy Encoding (.fastq)The identical reads and identical quality scores written with the legacy Phred+64 offset used by older Illumina pipelines. Decode it with the modern offset and every base looks 31 points better than it is, which is a silent quality inflation rather than a parse failure.
- Intentionally Corrupt FASTQ — Final Record Missing Its Quality Line (.fastq)An intentionally corrupt FASTQ whose first five records are complete and whose sixth ends after the plus line, leaving no quality string. A four-line-block reader must report an incomplete final record rather than pairing the sequence with an empty quality string.