HHMI and California Institute of Technology, Division of Biology and Biological Engineering, 1200 E California Blvd, Pasadena, 91125, USA.|HHMI and California Institute of Technology, Division of Biology and Biological Engineering, 1200 E California Blvd, Pasadena, 91125, USA.|HHMI and California Institute of Technology, Division of Biology and Biological Engineering, 1200 E California Blvd, Pasadena, 91125, USA.|HHMI and California Institute of Technology, Division of Biology and Biological Engineering, 1200 E California Blvd, Pasadena, 91125, USA. pws@caltech.edu.