Expression of exogenous mRNAs has become part of the standard approach to studying gene function during development of the sea urchin. The method is simple and reliable, protocols for the preparation of synthetic mRNAs are well described, and the technique to transfer them into eggs is efficient. The protein encoded by these mRNAs can be designed to address a variety of biological questions and their DNA matrices are easily constructed by standard molecular biology techniques. The method aims to simulate gain or loss of gene function, and the phenotypes obtained are characterized using an increasing number of molecular markers. With the completion of the S. purpuratus genome project, the complete set of genes from the sea urchin will become available. Expression of mRNA will be an invaluable tool to study the function of newly identified genes and their protein products and to determine their positions within the networks of gene and protein interactions that control development.