RESTRICTION ENDONUCLEASES ... . Restriction endonuclease K . . . . Restriction endonuclease B ... . Restriction endonuclease P .... , . R/ and Rll restriction endonucleases, .. . Hemophilus injiuenzae endonuclease .. , .... . THE ROLES OF SAM AND ATP IN RESTRICTION, , .. SAM." ... . . . .. . . . . . . . , . ATP .. " ,.,., ... . .. , . , . . . . .. , ... . MODIFICATION METHYLASES ... . Modification methylase B, .. . . . ..... , , , . . . Modification methylase P, . . . . .. ,. " ,. " " , .. " .... . ... .. . Modification methylase activity of the K restriction enzyme. , . . . . . . . . . . . . . . . Modification methylase activity of the PI restriction enzyme .. . An RJ modification activity . GENETIC ANALYSIS ., , , , . , . . . . . . . ....... . . ......... , Restrictionless mutants .. , .. Genetic mapping "."" , . . Complementation studies .... . . . . . . . ....... . MODIFICATION AND RESTRICTION SITES ON DNA . . . . . , . , . , . , , , , .. . Hemophilus endonuclease sites. , , , , , , , , , . . . . . . . . . .... . ..... . Sites of restriction and modification by E. coli enzymes . A speculation. . . . . . . . . . . . . . . . . . . . . . .. .