Electrophoretic mobility shift assay for DNA-protein binding. (A) 1nM of lox1 DNA was incubated with indicated amounts of Cre protein under the conditions described under ‘Methods’. cI and cII, DNA-protein complexes bound with one and two Cre molecules, respectively, at the loxP site. About 91% and 93% of the DNA were in cII when [Cre] was varied from 20nM to 200nM. (B) 1nM FRT1 DNA was incubated with indicated amounts of Flp protein. cI and cII, DNA-protein complexes bound with one and two Flp molecules, respectively, at the FRT site. About 83% and 91% of the DNA formed cII complex when [Flp] was varied from 20nM to 200nM.