Over- and under expression of the 22 kDa peripheral myelin protein (PMP22) results in dysmyelinating peripheral neuropathies, such as Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy, with the liability to pressure palsies (HNPP). Expression of the PMP22 gene is driven by two alternative promoters, P1 and P2, with transcripts originating from P1 associated with peripheral nerve myelination by Schwann cells. Transient transfections of constructs containing P1 (3.5 kb) or P2 (2.5 kb) resulted in high levels of reporter gene expression in the RT4-D6P2T schwannoma cell line. Serial deletions of P1 revealed that region P1-A (-105 to -43), situated upstream of the minimal promoter, contained a positive regulatory element. The 62 bp P1-A region conferred in cis a sevenfold increase in expression of luciferase driven by a heterologous promoter in an orientation-dependent manner. Interspecies comparison of the P1-A region revealed a 98% degree of identity between the human, mouse, and rat sequences. A prominent sequence-dependent DNA-protein complex (C-I) was detected in electrophoretic mobility shift assays with P1-A using RT4-D6P2T nuclear extract and was localized to a minimal 21 bp region within P1-A. Site-directed mutagenesis of this region revealed nucleotides at positions -46 to -43 as being necessary for formation of C-I. Functional analysis of the mutated P1-A element indicated that positions -46 and -45 were essential for transactivation mediated by this element. Characterization of the transacting factor(s) interacting with this key regulatory element will shed light on its role in regulating peripheral nerve myelination.