RNA ‘sponges’ mop up sequences that curb gene expression. Behold the latest curio in the cabinet of RNA oddities: naturally occurring circular RNA molecules that influence gene expression. At least some of the loops, described in two papers published this week by Nature1, 2, act as molecular ‘sponges’, binding to and blocking tiny gene modulators called microRNAs. But the researchers suspect that the circular RNAs have many other functions. The molecules comprise “a hidden, parallel universe” of unexplored RNAs, says Nikolaus Rajewsky, the lead author of one of the studies and a systems biologist at the Max Delbrück Center for...