As the battle over plant-based meat analogues heats up, one point of distinction between the leading contenders may be of importance: the presence or absence of genetically modified organisms (GMOs).

Impossible Foods has them; its main competitor, Beyond Meat, doesn’t. GMOs are an integral part of Impossible Foods products, because its main point of distinction is the use of “heme,” a form of hemoglobin usually derived from soy, that gives the Impossible Burger a semblance of meat juice. The founder of Impossible Burger discovered how to make heme from bioengineered yeast instead of soy plants, which helped make Impossible’s products commercially viable.

As Impossible products become more widespread – they’re now in fast food chains including Burger King – this use of bioengineering is becoming an increasing negative, CNBC reports. Environmental advocacy groups like the Non-GMO Project and Friends of the Earth have criticized Impossible Foods, especially in light of the company’s mission to reduce or even eliminate meat consumption for ecological reasons. A managing partner at PowerPlant Ventures, an investment firm that specializes in plant-based foods, told CNBC that that the GMO situation was one factor in his firm investing in Beyond Meat instead of Impossible Foods.

Impossible has stoutly defended its use of GMOs, touting the overall environmental benefits of its products and calling its critics on the GMO issue “anti-science.”