Expression of genes is manifested by the production of RNA transcripts within cells. Hybridization histochemistry (or in situ hybridization) permits localization of these transcripts with cellular resolution or better. Furthermore, the relative amounts of transcripts detected within different tissues or the same tissues in different states (e.g., physiological or developmental) may be quantified. This unit describes hybridization histochemical techniques using either oligonucleotide probes or RNA probes (riboprobes). Also presented is the use of probes labeled with digoxigenin for colorimetric detection of RNA transcripts and a technique to detect the Y chromosome using either mouse or human riboprobes. Finally, a procedure is presented for the autoradiographic detection of radiolabeled probes. Methods are provided for labeling oligodeoxynucleotide and RNA probes and performing northern analyses using these probes.