Alteration of gene expression can result in numerous pathologies, including proliferative diseases, functional deficits, and developmental defects. Recent studies have suggested that changes in the expression domains of Homeotic (Hox) genes during development are associated with the induction of chemically induced developmental anomalies. Before meaningful data on the alteration of gene expression can be obtained, the heterogeneity of normal expressions within a species must be evaluated. In situ hybridization is a powerful technique that can detect perturbations in the spatial and/or temporal expression of genes. The hybridization protocol described here utilizes 33P end-labeled, single-stranded DNA oligomeric nucleotides. Some of the technical advantages to this approach are that (1) DNA probes are not sensitive to RNase, (2) the probes are based on published sequences and are chemically synthesized, (3) probes are end-labeled utilizing a terminal transferase reaction, producing probes of high specific ...