The herpes simplex virus 1 (HSV-1) d106 mutant virus is a multiple immediate-early gene deletion mutant virus that has been effective as an AIDS vaccine vector in rhesus macaques (Kaur A, Sanford HB, Garry D, Lang S, Klumpp SA, Watanabe D, et al. Ability of herpes simplex virus vectors to boost immune responses to DNA vectors and to protect against challenge by simian immunodeficiency virus. Virology 2007;357:199-214). Further analysis of this vector is needed to advance development into clinical trials. In this study we have defined the precise nature of the multiple IE gene mutations in the d106 viral genome and have used this information to construct a new transfer plasmid for gene transfer into d106. We tested the effect of an additional mutation in the U(L)41 gene on d106 immunogenicity and found that it did not improve the efficacy of the d106 vector, in contrast with results from other studies with U(L)41 gene mutants. The safety profile of d106 was improved by generating a new vector strain, d106S, with increased sensitivity to acyclovir. Finally, we have constructed a d106S recombinant vector that expresses the HIV clade C envelope protein. The d106S HIVenvC recombinant has retained the sensitivity to acyclovir, indicating that this phenotype is a stable property of the d106S vector.