Peripherin/RDS is a member of the tetraspanin family of integral membrane proteins and plays a major role in the morphology of photoreceptor outer segments. Peripherin/RDS has a long extracellular loop (hereafter referred to as the LEL domain), which is vital for its function. Point mutations in the LEL domain often lead to impaired photoreceptor formation and function, making peripherin/RDS an important drug target. Being a eukaryotic membrane protein, acquiring sufficient peripherin/RDS for biophysical characterisation represents a significant challenge. Here, we describe the expression and characterisation of peripherin/RDS in Drosophila melangolaster Schneider (S2) insect cells and in the methylotrophic yeast Pichia pastoris. The wild-type peripherin/RDS and the retinitis pigmentosa causing P216L mutant from S2 cells are characterised using circular dichroism (CD) spectroscopy. The structure of peripherin/RDS and of a pathogenic mutant is assessed spectroscopically for the first time. These findings are evaluated in relation to a three-dimensional model of the functionally important LEL domain obtained by protein threading.