Ambion® RNA Millennium™ markers are designed to determine the size of single-stranded RNA transcripts from 0.5 to 9 kb and can be used in Any Northern blot hybridization protocol. Sufficient markers are provided for 25 Northern blot hybridization gel lanes. They consist of a mixture of 10 different RNA transcripts with easy-to-remember sizes: 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6 and 9 kilobases. These markers can be stained with ethidium bromide during or after electrophoresis. Due to their uniform number and spacing, these markers are ideal for constructing accurate standard curves and determining mRNA size. They also provide a reference scale that allows easy verification and comparison of known bands. Using ethidium bromide staining, 1–2 μg (1–2 μg) of RNA Millennium™ marker produced 10 distinct bands on a 1% denaturing agarose gel. All marker lots undergo stringent nuclease testing and are stable overnight at 37°C.