Diagnostics powerhouse Danaher Corp. has agreed to buy Aldevron LLC, a privately held biotech company that supplies a key ingredient for COVID-19 vaccines, from EQT VIII Fund for $9.6 billion in cash. Based in Fargo, N.D., with additional operations in Madison, Wis., Aldevron makes high-quality plasmid DNA, mRNA and recombinant proteins used in vaccines, gene and cell therapy, gene editing and diagnostic applications.