General Feature Format, commonly referred to as GFF, is a standardized file format designed for representing genomic and functional annotations. It serves as a critical bridge between raw sequence data and biological interpretation, allowing researchers to store information about genes, transcripts, proteins, and other genomic features in a consistent, tab-delimited structure. This universality is the format’s greatest strength, ensuring that data generated by one tool or organization can be seamlessly imported and visualized in a wide array of bioinformatics software and genome browsers.
The Core Structure and Specifications
At its heart, a GFF file is a plain text document organized into columns that follow a strict hierarchy. Each line, except for comment lines starting with a hash, represents a distinct genomic feature. The structure relies on a zero-based coordinate system for start positions and a one-based coordinate system for end positions, a nuance that is vital for correct data handling. The format specifies nine mandatory columns that control the fundamental identity and location of the feature, while allowing for an expansive "attributes" column that functions as a key-value store for metadata.
Deconstructing the Nine Columns
The first seven columns form the foundational definition of any feature, while the eighth and ninth provide flexibility and detail. Understanding these columns is essential for anyone working with genomic data.
Column | Description | Example
1. Source | The program or dataset that generated the annotation. | Ensembl, NCBI, Augustus
2. Type | The feature category, such as gene or exon. | gene, mRNA, CDS
3. Feature | A specific name or identifier for the feature. | ENSG00000139618
4. Start | The starting position of the feature. | 1000
5. End | The ending position of the feature. | 2000
6. Score | A numerical value indicating confidence or quality. | 0.95
7. Strand | The DNA strand on which the feature resides. | + or -
8. Phase | For coding features, the reading frame offset. | 0, 1, or 2
9. Attributes | Additional metadata in semicolon-separated key-value pairs. | ID=gene123;Name=BRCA2
