Involves amplification by transcription of cdna into crna (requires less starting material) Probes consists of mis-match (mm) and perfect match (pm) types. Mismatch, one of the nucleotides on the probe is off (not complementary) everything else is. Probe is entirely complementary to the sequence of the gene you"re looking at (crna) Sample is hybridized at a specific temperature where sample will bind to pm and not mm probes. Hybridized at a specific temperature so that crna will bind to the pm probe. High hybridization ratio of pm to mm indicates expression of a gene. More crna is expected to bind to the pm probe than the mm probe, this way you get a higher hybridization ratio. 25 base pair probes are less specific and require multiple probes/genes to compensate. Probes are 25-mers, so they are less specific. Because there is a higher density of the probe, you can use more of them.