pOG44 is a 5785 bp vector that expresses a temperature-sensitive Flp recombinase
(flp-F70L) under the control of the human CMV promoter as previously described
(O'Gorman et al., 1991). The vector contains a synthetic intron to enhance
expression of the FLP gene.
 Note that the vector does not contain an antibiotic resistance marker to allow 
stable selection in mammalian cells.