Lillian Laemmle Email: Lll19@pitt.edu Lab: Glorioso lab: HSV-1 gene expression occurs in a cascade fashion starting with the immediate early genes. The immediate early proteins ICP0, ICP4, and ICP27 activate the transcription of early and late genes, however ICP4 and ICP27 alone are required in vitro for viral replication. Deletion of either ICP4 or ICP27 from HSV-1 creates a virus that is replication-defective, a fact which has proven to be extremely useful in the creation of vectors for use in a variety of applications. The promoter sequence of Pax3, a murine gene that encodes a transcription factor involved in the development of limb muscles, has been engineered in front of the coding sequence of ICP4 into an ICP4 deleted replication defective form of HSV-1. This vector should be able to replicate only in cells where the Pax3 promoter is activated, and will be used to study factors involved in myogenesis.