what the difference bet. mRNA, CDS and exon? - (Oct/07/2010 )

Dear All
PLEASE i want to know if i want to design a primer for PCR and RT-PCR
when i search for the nucleotide sequence on Genbank i found 3 expressions (mRNA - CDS - Exon) and the same sequence for them all are the same
so ARE the primers for PCR applied also for RT-PCR at the same time???
and if they didn't tell the positions of the exons how could i know them???

Thank you veryy much

genearlly speaking, they are the same. mRNA sequences exist in the database as CDs (coding sequences). Exons are also coding sequences.