The methylotrophic yeasts Hansenula polymorpha, Pichia pastoris and Candida boidinii have been developed as production systems for recombinant proteins. The favourable and most advantageous characteristics of these species have resulted in an increasing number of biotechnological applications. As a consequence, these species - especially H. polymorpha and P. pastoris - are rapidly becoming the systems of choice for heterologous gene expression in yeast, Recent advances in the development of these yeasts as hosts for the production of heterologous proteins have provided a catalogue of new applications, methods and system components.