Phage vectors are often used rather than plasmids, particularly for the production of gene "banks" or "libraries." The plaques produced can be screened for the presence of specific DNA sequences by hybridization using a procedure similar to that used for colony hybridization (see Chapter 42 ). Plaque hybridization offers some advantages over colony hybridization, largely because the area of the filter to which the DNA is bound is smaller and more defined. A higher density of plaques can therefore be used, which in turn means that more plaques can be screened in a single hybridization-several thousand on an ordinary (8.5 cm) petri dish; using larger containers such as baking dishes or trays the number can run into hundreds of thousands in a single hybridization. In addition, since the location of the plaques is not disturbed by the process, nor are they smudged in the way that colonies can be, several replicates can be taken from the same plate. This means that the same set of plaques can be screened for hybridization to several different probes.