Gene Expression Data Format

Coding Strand

Matches the mRNA nucleotides (with U's instead of T's).

Template Strand

Read by RNA Polymerase when it's creating the mRNA.

If you have a transcript of mRNA, you can map it back to the gene it came from based on the sequence.
You can sequence all the mRNA content in a cell, and use that to quantify the expression level of genes.¹