The deciphering of complete genome sequences has opened a post-genomic proteomics era. While the sequence of many proteins is now known, attention will increasingly focus on the complex interaction networks that regulate their activity, and the analysis of protein distribution in the cell will be crucial to elucidating their function. A new generation of gene trapping devices, protein trap transposons, offers a way of analysing In vivo the dynamics of sub-cellular distribution of a large number of proteins. Many transgenic lines have already been established and are available. Screens focusing on particular cell compartments can be devised.