Integrase inhibitors are emerging anti-human immunodeficiency virus (HIV) drugs, and multiple retroviruses and transposable elements were evaluated here for susceptibilities to raltegravir (RAL) and elvitegravir (EVG). All viruses, including primate and nonprimate lentiviruses, a Betaretrovirus, a Gammaretrovirus, and the Alpharetrovirus Rous sarcoma virus (RSV), were susceptible to inhibition by RAL. EVG potently inhibited all lentiviruses and intermediately inhibited Betaretrovirus and Gammaretrovirus infections yet was basically ineffective against RSV. Substitutions based on HIV type 1 (HIV-1) resistance changes revealed that integrase residue Ser150 contributed significantly to the resistance of RSV. The drugs intermediately inhibited intracisternal A-particle retrotransposition but were inactive against Sleeping Beauty transposition and long interspersed nucleotide element 1 (LINE-1) retrotransposition.Reverse transcription of retroviral RNA yields linear viral DNA (vDNA) containing a copy of the long terminal repeat (LTR) at each end. Integrase (IN) is an essential retroviral enzyme that catalyzes two reactions to insert the vDNA into cellular chromosomal DNA. IN prepares the LTR ends by hydrolyzing phosphodiester bonds adjacent to invariant CA dinucleotides, yielding reactive 3Ј deoxyadenylate (dA OH ) termini. In the nucleus, IN catalyzes DNA strand transfer by using the 3Ј OHs to cut the chromosome in a staggered fashion, concomitantly joining the vDNA ends to 5Ј phosphates. Hostmediated repair of the resulting DNA recombination intermediate completes the integration process. See reference 8 for an overview of retroviral reverse transcription and integration.IN belongs to the polynucleotidyl transferase superfamily of nucleic acid-metabolizing enzymes (7). Conserved amino acid residues (typically Asp and Glu [32]) arranged commonly on an RNase H structural fold comprise active sites that coordinate divalent metal ions for in-line nucleophilic attack of phosphodiester bonds. Due to its critical role in replication, human immunodeficiency virus type 1 (HIV-1) IN has long been targeted for drug development, and the first-in-class inhibitor raltegravir (RAL) was licensed in 2007 (45). Because RAL and related compounds preferentially inhibit DNA strand transfer activity, the drugs are referred to as IN strand transfer inhibitors (INSTIs) (24). Elvitegravir (EVG) is another well-studied INSTI (41). Recently determined X-ray crystal structures revealed that the drugs work by ejecting the 3Ј dA and its associated OH nucleophile from the IN active site (10,