Yeast expression systems, including the methylotrophic yeast Pichia pastoris, have been used over the years as powerful expression systems for a number of heterologous genes. The pPICHOLI vectors have been designed for heterologous gene expression in P. pastoris as well as in E. coli. This prokaryotic expression system avoids labor-intensive and costly subcloning procedures and allows for high-level production of heterologous proteins. The PORF system provides improved cloning of open reading frames in yeast.