Replication-deficient genomic HSV vectors have been used for transfer of genes to a variety of cell types including neuronal cells, embryonic stem cells as well as adult precursors from human adipose tissue. Human adipocytes and their precursors (Zen-Bio Inc.) are both excellent targets for HSV-mediated transduction and transgene expression (Fradette et al. 2005). Similarly, murine embryonic stem cells express HSV entry receptors, leading to more than 90% of transduced cells at low multiplicity. However, highly defective HSV vectors (e.g. QOZHG) deleted for the majority of immediate early genes including ICP4, ICP27 and ICP22 remain cytotoxic in non-neuronal cells, resulting in the majority of precursor cells dying within 4-5 days post-infection. Therefore, when induced towards differentiation, QOZHG-infected embryonic stem cells do not efficiently generate embryoid bodies (<35%), while preadipocytes are unable to differentiate into lipid-filled adipocytes. Vector cytotoxicity has been linked to expression of the remaining IE gene product encoded by ICP0. ICP0 is a ubiquitin ligase that prevents vector genome silencing and overcomes the cells anti-viral state in response to virus invasion. Here, we report that a derivative of the vector TOZHG.2 has been altered in its toxic effects on both embryonic and adult adipose stem cells while maintaining high levels of eGFP transgene expression following low multiplicity of infection. For example, at least 70% of human preadipocytes are transduced by TOZHG.2, displaying transgene expression for up to 30 days in culture. Upon infection with this vector, these cells preserved their ability to respond to differentiation signals and gave rise to mature adipocytes as effectively as uninfected cells (100 +/|[minus]|0 10%), as quantified by Oil Red O staining, a measure of lipid accumulation into cells. Many GFP positive adipocytes (40% of transduced cells) were observed for as long as 31 days after infection whereas none were seen among QOZHG-infected cultures. In a similar fashion, TOZHG.2-transduced embryonic stem cells were efficiently transduced (99%) and formed embryoid bodies at greater than 70% of the levels seen for mock-infected cells. The precise reason for the lack of apparent cytoxicity of this peculiar recombinant virus is to be determined but may be related to ICPO expression, since 10.0 +//0 1.5 lower ICPO protein levels are expressed by TOZHG.2-transduced cells compared to QOZHG-infected ones. In addition to this reduction in ICPO expression levels, we are currently investigating whether an alteration of ICPO's structure or biological function may explain the lack of cytotoxic effects of this vector derivative.