A gain-of-function cell-based assay to characterize the mode of action of small chemical compounds. (a) A collection of 12,212 hORFs was cloned en masse into the lentiviral expression vector pLD-IRES-Venus-WPRE-STOP. The resulting constructs were used to produce lentivirus and infect the rtTA-expressing cell line HEK293_M2 at a multiplicity of infection (MOI) of 0.3. Sorting of the Venus-positive cells resulted in the isolation of human cells with the functional integrated lentiviral constructs. (b) After seeding, doxycycline (dox) was added to the cells in order to induce the expression of the hORFs. After selection in the presence of a lethal dose of drug, surviving cells were harvested, gDNA extracted and the nature of the hORFs providing resistance to the chemical identified after hybridization on microarray. NGS, next-generation sequencing.