This invention relates to functional gene arrays of yeast. Novel aspects include the individual yeast cells, methods for making the yeast and the arrays, the arrays and uses for the arrays. A diploid bearing special genetic properties has been constructed to facilitate cloning of heterologous genes capable of providing essential functions. A selection method, using this strain allows the identification haploid yeast strains dependent for life on heterologous essential genes. The arrays of these strains comprise a library of unique members where each member is dependent for survival on the function of a heterologous gene complementing a different essential host gene which has been inactivated by the insertion of a dominant selectable marker. These arrays provide screening platforms for agents that specifically target the activity of these heterologous genes.