The current shRNA library was made in the laboratory of Greg Hannon in the pSHAG-MAGIC2c vector (short hairpin activated gene silencing - Mating Assisted Genetically Integrated Cloning System), which is a modified version of the original pSHAG-MAGIC vector constructed in the laboratory of Stephen Elledge. Both vectors are self-inactivating stem cell virus (MSCV) retroviral vectors containing a number of convenient design features to allow in vivo DNA transfer from the donor (the shRNA containing) vector to a recipient vector. The modified vector permits the insertion of the PCR amplified oligo containing the hairpin and common 5' and 3' ends. PGK-puro - murine phosphoglycerate kinase (PKG) promoter used to express the puromycin resistance gene for antibiotic selection in eukaryotic cells. Source.