Data matrix expression file is a Tab-separated values file.

A tab-separated values (TSV) file is a simple text format for storing data in a tabular structure (e.g. database or spreadsheet data). Each record in the table is one line of the text file. Each field value of a record is separated from the next by a tab stop character. More about TSV...

This file has two columns if there is only one sample, and more than two columns if there are many samples. The first line is a header and must contain the sample names. The first column correspond to gene, probe or protein Ids from the following databases:

1. Ensembl gene
2. HGNC symbol
3. Entrez id
4. Affy HG U133A probeset
5. Affy HG U133B probeset
6. Affy HG U133-PLUS-2 probeset
7. Affy HTA 2.0

The next columns correspond to gene expression values in numeric format from each sample.

Here is an example of a file with only one example:

expression.txt

id	sampleName
1	0.3
2	1
3	0.73

And here is another example with more than one sample:

expression-multi.txt

id	sample1	sample2	sample3
1	0.31	0.6	0.24
2	1	0.81	0.91
3	0.7	0.9	0.3
4	0.23	0.45	0.33

You can download an usable example file from Here