Proposed Model for the Rescue and Replication of LuIII Lt-Lt. Restriction sites and their positions with respect to the LuIII sequence are indicated. Grey, white and zigzag regions represent pUC19, LuIII left terminus and E. coli sequences respectively. In steps 4 and 5 the molecules generated (a/b and aa/bb) are identical, for this reason only one molecule at each step is continued throughout the scheme. The estimated sizes of some of the molecules (boxed) are indicated.