Precursors of most secreted and cell surface molecules carry signal sequences at their amino termini. The method coined as signal sequence trap (1–3) takes advantage of the presence of N-terminal signal sequences in most precursor forms of secretory proteins and transmembrane proteins, which are necessary for the proper orientation of the N-terminal of mature forms inside endoplasmic reticulum and exocytotic vesicles. This method enables to selectively clone cDNA species encoding intercellular signal-transducing molecules without biologic assays. In this chapter, an efficient signal sequence trap method based on an Epstein-Barr virus shuttle vector is described (2).