AbstractA gene‐trap strategy was set up in embryonic stem (ES) cells with the aim of trapping genes expressed in restricted neuronal lineages. The vector used trap genes irrespective of their activity in undifferentiated totipotent ES cells. Clones were subjected individually to differentiation in a system in which ES cells differentiated into neurons. Two ES clones in which the trapped gene was expressed in ES‐derived neurons were studied in detail. The corresponding cDNAs were cloned, sequenced, and analysed by in situ hybridisation on wild‐type embryo sections. Both genes are expressed in the nervous system. One gene, YR‐23, encodes a large intracellular protein of unknown function. The second clone, YR‐14, represents a sorting nexin (SNX14) gene whose expression in vivo coincides with that of LIM‐homeodomain Islet‐1 in several tissues. Sorting nexins are proteins associated with the endoplasmic reticulum (ER) and may play a role in receptor trafficking. Gene trapping followed by screening based on in vitro preselection of differentiated ES recombinant clones, therefore, has the potential to identify integration events in subsets of genes before generation of mouse mutants. © 2001 Wiley‐Liss, Inc.