Since 1925, commercial "Cucumis melo" (cantaloup and muskmelon) production has been engaged in a biological "arms race" against cucurbit powdery mildew (CPM) caused by the fungus "Podosphaera xanthii", with new cultivars of melons being developed for resistance to successively arising races of the fungus, identified simply as race 1, race 2, etc. (seven in total by 2004), for races found around the world, and race N1 through N4 for some divergent races native to Japan. Various subraces have been identified, and given names such as race 2U.S., race 3.5, and race 4.5. A new race S was discovered in 2003, and a specific melon cultivar ("C. melo" var. "acidulus" 'PI 313970') found resistant to it, then used for backcrossing to increase resistance in other cultivars. Such modern selective breeding of plants for phytopathological resistance to particular fungal races involves a great deal of genetic research; this PI 313970 versus race S case involved multi-stage hybridization to propagate a recessive gene, "pm-S" in successive generations, and how this may affect other recessive and codominant genes for resistance to other races of "P. xanthii" "remains to be determined".