Why Is Quantum-Si Incorporated ( $QSI) Stock Down -15.38% Today? (Find out why any stock moved today on Tickeron.) QSI shares were down roughly 15.4% intraday, retreating to about $1.21 from a prior close of $1.43. The decline follows an explosive multi-day rally that lifted the stock more than 80% in about a week, including a 33.6% surge in the previous session. The move is widely attributed to profit-taking and consolidation rather than any new company-specific negative headline. $PACB $AXON $VCYT $BFLY $MESO