Novel genes encoding P. pastoris ARG1, ARG2, ARG3, HIS1, HIS2, HIS5 and HIS6 are disclosed. A method for inactivating alternately at least two biosynthetic pathways in a methylotrophic yeast is provided. A method for producing and selecting yeast strains characterized as being capable of genetic integration of heterologous sequences into the host genome using the genes involved in the biosynthetic pathways is also disclosed.