Insights into the mechanisms derived from these structures are explained later.Cre is an enzyme encoded by phage P1, which functions to circularize the linear phage genomeduring infection. The recombination sites on the DNA, where Cre acts, are called lox sites.Cre–lox is a simple example of recombination by the tyrosine recombinase family; only Cre protein and the lox sites are needed for complete recombination.Cre is also widely used as a tool in genetic engineering (see Box 12-1, Application of Site-Specific Recombination to Genetic Engineering).