Over the last several decades, an understanding of the events involved in embryonic pattern formation has been extended from an anatomic to a molecular level. In particular, the ability to examine the spatial and temporal expression of specific genes has aided greatly the analysis of their function. One of the most powerful techniques for determining gene-expression patterns isin situhybridization, a method that relies on the hybridization of labeled antisense RNA probes to detect endogenous RNA transcripts at their sites of expression (1 ).
