Interview with Karen N. Allen, Boston University, USA Karen Allen is talking with Martin Hicks about the functional analysis and impact of the ‘promiscuity’ of enzymes. Enzymologists use this term to describe the ambivalency of enzyme activity. Either these enzymes carry out different but related chemical reactions or they catalyse a different set of substrates. Karen describes the significance of promiscuous enzymes for the evolutionary development of enzymes when selectivity and efficiency is optimized and underlines that promiscuity is still of importance for the adaptation of organisms to changing environments. She also points out that promiscuity are a considerable challenge for drug discovery since either these enzymes circumvent drug effects or even metabolize drugs as alternative substrates. However, the more enzymologists study the dynamics of enzymes the more promiscuity is found among a wide variety of enzyme families.