Selection of recombinant clones necessitates the use of an appropriate selectable marker system.
- screening by vector molecules which includes antibiotic resistance genes or β-galactosidase gene complementation
- Generalized recombinant screening by insertional inactivation. This can be achieved by β-galactosidase screens or suppressor t-RNA-based screens.
- Directed recombinant screening. This can be achieved by hybridization-based screening by using labeled probes or by using PCR-based screening. 
4. Cloning systems for different sized DNA fragments:
Such cloning systems normally include an antibiotic resistance gene (to enable screening for presence of vector) and a marker gene with a multiple cloning site (to enable screening recombinant clones).
See Table 5.2 for different cloning vectors and the DNA insert sizes that each could accommodate. 
Lambda and cosmid vectors are used in cloning moderately large DNA fragments in bacterial cells.
Three types of λ derived cloning vectors:
a. Replacement λ vectors: removal of central section of the genome and replacing by a foreign DNA fragment (up to 23 kb inserts)
b. Insertion λ vectors: modifications to allow insertional cloning of cDNA fragments into the cI gene (up to 5 kb)
c. Cosmid vectors: cos sequences of λ are inserted into a small plasmid generating a cosmid. Can take 33 – 44 kb inserts.
Bacterial Artificial Chromosome (BAC) vectors are used to clone large fragments (>300 kb). Low copy number (1-2 copies/cell) fertility factor (F-factor) plasmids are used for this purpose.
Bacteriophage P1 vectors and P1 artificial chromosomes (PACs). Components of the P1 phage are included in a circular plasmid and can accommodate up to 122 Kb DNA fragments.
