Design of the cDNA expression vector and the conditional expression vectors. (a) Map and multiple cloning sites of expression vector pExpress. After cloning of the cDNA coding sequence into pExpress, the expression cassette can be excised as a SpeI cassette. Location of forward and reverse primers are shown by triangles. (b) Insertion of the cDNA expression cassette into the mutant loxP vectors. Cloning of the expression cassette (SpeI cassette) into the SpeI site of mutant loxP vectors (pLoxNeo, pLoxPuro and pLoxBsr) produces an expression vector whose selectable marker can be recycled. Cloning of the expression cassette (SpeI cassette) into the NheI site of mutant loxP vectors (pLoxPuro and pLoxBsr) produces a vector in which a selectable marker and an expression cassette is flanked by mutant loxP sites.