Coding sequence is the portion of the gene or the mRNA which actually codes for a protein. Introns are not coding for the sequences; nor are the 5' or 3' untranslated regions (or flanking areas, for that matter - they are not even transcribed into the mRNA). The coding sequence in the cDNA or mature mRNA includes everything from AUG (or ATG) initiation codon through to the stop codon.