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'Downstream' refers to a relative position in DNA or RNA. Each strand of DNA or RNA has a 5' end and a 3' end, so named for the carbons on the deoxyribose ring. Relative to the position on the strand, downstream is the region towards the 3' end of the strand. Since DNA strands run in opposite directions, upstream on one strand is downstream on the other end.

Another important meaning of the term "downstream", refers to the temporal and mechanistic order of cellular and molecular events. For example, in signal transduction, the second messenger acts downstream to activation of cell membrane receptors.