Progress in finding resistance in other cucurbit species has not been as successful. Even though resistance has been found in other Cucurbit species, genetic incompatibility among species remains a formidable barrier for transferring this resistance to Cucurbita pepo crops, such as summer squash. The precocious gene, which masks color breaking in yellow summer squash for CMV (and WMV), is present in many yellow straightneck and crookneck varieties, and several have transgenic resistance or intermediate resistance for CMV. Insertion of the precocious gene is by traditional cross breeding whereas transgenic resistance involves the insertion of one or more foreign genes from a different organism. A handful of zucchini squash varieties have intermediate resistance for CMV and a few have transgenic resistance.