In the peripheral nervous system the myelin sheath is produced by the spiral wrappings of the Schwann cell (SC) membrane around the axon. This provides insulation and increases the velocity of impulse propagation. The structure of myelin is maintained by a group of myelin proteins. Peripheral myelin protein-22 (PMP-22) is a 22 KDa glycoprotein, originally identified following nerve crush injury, that is found within SCs and is identical to the growth arrest specific protein GAS-3. The PMP-22 gene is regulated by two alternative promoters immediately upstream of two alternative non-coding exons. In order to study temporal and spatial expression of the PMP-22 gene and regulation of SC ensheathment and myelination, a transgenic mouse expressing the green fluorescent protein (GFP) driven by the myelin specific PMP-22 promoter was produced. To achieve this the PI promoter isolated from genomic DNA was initially incorporated into a plasmid containing the EGFP gene. In vitro transfection studies demonstrated appropriate expression of EGFP fluorescence. Microinjection of the transgene into pre- implantation fertilised embryos gave rise to three transgenic lines as confirmed by Southern blot and PGR. One founder expressed the transgene in a tissue specific manner. Mosaicism of expression both within an individual and between individuals was noted. In vitro manipulations showed that the expression patterns observed were independent of axonal contact and myelination but could be influenced by the extracellular matrix. These GFP expressing transgenic mice potentially provide a means to determine the dynamics of SC-axon interactions during myelination and the behaviour of transplanted SCs into myelin deficient regions and the SCs response to injury. Preliminary reports of this work are found in abstract form: British Neurosci. Assoc. Abstr., Vol 15, pi04, 1999.