Yeast artificial chromosome (YAC) libraries stored in microtiter plates are available for screening as either complex PCR pools or hybridization filters generated from YACs gridded at high densities (see Chapter 3 ). Different libraries may be available as either PCR pools, hybridization filters, or both. Consequently, screening strategies have been designed that rely solely on either technique, or use a combined approach. Clearly, access to the YAC library microtiter plates and an automatic gridding system allows the user greater flexibility and is an advantage in large mapping projects.