Lambda DNA, a linear, double-stranded phage DNA containing 12 bp single-stranded complementary 5"-ends, is derived from an Escherichia coli bacteriophage (Bacteriophage lambda cI₈₅₇ Sam7). Lambda DNA (48,502 bp) may be used as a molecular weight size marker during nucleic acid gel analysis following digestion with a restriction enzyme (such as HindIII). Lambda DNAcan also beused as a substrate in restriction enzyme activity assays.