The identification of novel, developmentally regulated genes whose products play roles in the differentiation of specific vertebrate tissues and organs can be accomplished using a method called gene trapping (1 -9 ). This technique involves inserting a marker gene, such as β-galactosidase or alkaline phosphatase, into the genome of murine embryonic stem (ES) cells using one of several available methods of transfection. The gene trap construct contains an antibiotic resistance gene such as neomycin to permit the selection of transfected clones. ES cell clones are picked and expanded on multiwell plates before freezing. Screening of the clones for tissue-specific gene expression involves thawing multiwell dishes, expanding them, and allowing them to differentiate in vitro into multiple cell lineages (10 -14 ).
