The discovery of restriction and modification enzymes, which proved to be a major turning point in the progress of molecular biology, was a consequence of a bacteriological observation in the early 1950s (Luria and Human, 1952; Bertani and Weigle, 1953). The two groups reported the curious behaviour of phage grown on two different strains of bacteria. Phages propagated on one strain were found to grow poorly on the second (hence the term ‘restriction’) and vice versa. However, the few phages that escaped restriction could then grow well on the new host, thus being modified in a way that afforded them protection from the restriction imposed by the host.