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lthough gammaretroviruses have been hugely im-portant for proof-of-principle demonstrations of efficacyin several specific clinical applications, other retroviral clas-ses, lentiviruses in particular, are emerging as promisingalternatives. Of the on-going 35 trials using lentivirus-basedvectors (LVs), 24 are in phase I, 9 in phase I/II, and 1 each inphases II and III (source: http://clinicaltrials.gov/). In ad-dition to their ability to stably integrate into the genome oftarget cells for long-term expression of the gene of interest,LVs enjoy a few significant advantages through their abilityto transduce nondividing cells and also to allow robusttransgene expression in cells where there is a high proba-bility of epigenetic silencing or position-related variation ofactivity (Trono, 2000; Schambach and Baum, 2008). Early-generation gammaretroviral vectors have been shown to beparticularly prone to insertional mutagenesis through inad-vertent long terminal repeat (LTR)-mediated upregulationof proto-oncogene expression (Ott et al., 2006; Hacein-Bey-Abina et al., 2008; Howe et al., 2008; Stein et al., 2010). Inpart, this risk may be enhanced by the intrinsic preference ofthese vectors to integrate in and around gene regulatoryregions. LVs may therefore have an overall safer integrationprofile, although both exhibit a semirandom pattern of in-sertion into the human genome, and this type of risk stillcannot be ignored (Bushman et al., 2005; Cavazzana-Calvoet al., 2010). The safety of LVs has been enhanced in severalways including self-inactivating (SIN) vector design, tat in-dependence, use of insulators, and employment of internaltissue- or lineage-specific promoter and enhancer sequences,thereby reducing risks of insertional mutagenesis and vectormobilization (Matrai et al., 2010). Even so, longer term ob-servations in the clinical arena will be needed to define therisk for newer integrating vectors with more clarity. Con-currently, the development of a stable packaging cell line toensure useful batch consistency and size during large-scaleclinical vector production and purification under strict GoodManufacturing Practice (GMP) conditions has been at-tempted by various groups. Inducible systems for generatingstable cell lines have been developed to eliminate cytotox-icity from viral gene products required for packaging, suchas the HIV-1-derived protease and Rev, and the heter-ologous VSV-G envelope protein (Xu et al., 2001; Ikeda et al.,2003; Cockrell et al., 2006; Broussau et al., 2008; Kutner et al.,2009; Throm et al., 2009). It is also noteworthy thatreplication-competent LVs (generated by homologous re-combination between packaging and vector constructs) havenot been detected in large-scale production batches based onlatest generation packaging systems and vector design(Merten et al., 2011). In this commentary, we comment on sixoriginal research articles addressing three separate areasrelating to continuing efforts in improving retroviral design,production, and application for preclinical research andclinical applications.In this issue of Human Gene Therapy, Marc Giry-Laterrie`reand colleagues describe a modified tetracycline (TET)-inducible, polycistronic vector system encoding a selectablemarker and allowing for simple transgene cloning. As notedin their paper, the individual features incorporated in the‘‘polyswitch vector’’ have been previously described byseveral groups. The novelty lies in the combination of thesein a single vector to facilitate stable, inducible, and reversibletransgene expression from a single integration event. Sincethe description of the first tetracycline-inducible system byGossen and Bujard (1992), various small-molecule inducersystems have been employed to control the levels and timingof transgene expression both in vitro and in vivo (Vigna et al.,2002; Sirin and Park, 2003; Galimi et al., 2005). However,many have suffered from either low levels of inducibility orleakiness in the absence of the specific inducer. The modifiedpTF promoter used in this current study was shown to havelow basal activity and high inducibility. The authors alsoseparated a selectable marker (regulated by a ubiquitoushuman elongation factor-1a promoter) from the inducibletransgene cassette for enrichment of transduced cells beforeactivating the transgene of interest, and were able to dem-onstrate regulated expression from their vector in cell lines aswell as primary human cells. Of course, it will now beimportant to evaluate the performance of this vector inpreclinical animal models, especially in terms of drug in-ducibility and reversibility on withdrawal. Although this islikely to be useful for a number of applications, there willalso be restrictions on the nature of the transgene dictated

bsenceofeffectiveandreliableanimaland cellularmodels for studying neurodevelopmental and neurode-generative disorders such as Parkinson’s disease, Alzhei-mer’s disease (AD), Huntington’s disease, amyotropic lateralsclerosis, spinal muscular atrophy, and many others has se-verely compromised both basic and translational research inthe arena of such complex disorders. However, recent ad-vancements in the field of cellular reprogramming haveprovided an effective tool to study such disorders utilizingpatient-derived cells. Through ectopic overexpression of aFIG. 1. Immunophenotypic characterization of neuronal precursors and progenitors emerging out of trisomy 21 amnioticfluid–derived induced pluripotent stem cells (iPSCs). Nes, nestin; Tuj1, tubulin beta-III.