cDNA is single-stranded DNA that has been reverse-transcribed in vitro from cellular mRNA. By soaking a cDNA microarray with labeled RNAs, it is possible to determine which genes are active in a particular cell. A cDNA microarray is a grid of dots, with each dot containing a different cDNA sequence. In the figure below, darker shades represent a greater concentration of labeled RNAs that have hybridized to the cDNAs of a particular microarray dot. These sorts of experiments laid the groundwork for the theory that transcriptional control (as opposed to translation control) determines which genes are expressed, since cells not expected to express certain genes never even produces those RNAs (which be present unless expression was halted at transcription).