Abstract: Provided are microarrays comprising spots comprising mixtures of cDNA molecules, the cDNA mixture being complementary and substantially quantitatively proportional to a mixture of mRNA molecules present in a cell or group of cells. Also provided are methods for determining expression of a gene in a cell or group of cells using the invention microarrays. Additionally provided are methods of determining the difference in expression of a first gene between a first cell or group of cells and a second cell or group of cells, using the invention microarrays. Also provided are microarrays comprising short RNAs, and methods of using these microarrays for detecting and quantifying microarrays in cells.