If you are using a vector with lacZ as a screenable marker, which of the following bacterial strains would you want to use as host cells?
1. lacO^-
2. lacI^-
3. lacZ^-
4. Any of these would work.

In genetic engineering, the antibiotic-resistance gene is used:
1.  To select healthy vectors
2.  As sequences from where replication starts
3.  To keep the cultures free of infection
4.  As selectable markers