Differential gene expression is an essential mechanism for the development of a single-celled zygote into a patterned multicellular organism with differentiated cell types and tissues. Therefore, the identification of differentially expressed genes provides an important molecular resource for functional studies to define genetic pathways in metazoan development. Differential hybridization or subtraction methods have been used to detect differences in gene expression between distinct cells or tissues. However, these methods are difficult to apply to early stage embryonic tissues because they require relatively large amounts of tissue, cells and mRNA (1 -3 ).
