Urine gets its yellow hue from a microbial enzyme known as bilirubin reductase, according to a new study. While scientists identified the yellow pigment in urine, urobilin, more than 125 years ago, the enzyme or enzymes that makes urobilin remained a mystery until this study. The findings pave the way for research into the gut microbiome's role in adverse conditions like jaundice and inflammatory bowel disease.