A new study has revealed that a portion of ground beef sold in the United States isn’t 100 percent beef, reported AllGov.com. Some of the product contains horse, which is illegal to sell for consumption in America.

Chapman University’s Food Science Program conducted a study using 48 meat samples from different local supermarkets and tested the DNA traces of each sample. Ten out of 48 meat samples were mislabeled, nine of which contained different species of animals. Two samples included horse meat.

Dawn Kane, graduate research assistant at Chapman University, said the mislabeling “appears to be due to either intentional mixing of lower-cost meat species into higher-cost products or unintentional mixing of meat species due to cross-contamination during processing.”

Such mixing of animal species seems to be more common than the general public is aware. Another similar study involving 54 game meat samples found that 10 of those samples were mislabeled. Two samples labeled as bison and one as yak were actually domestic cattle.