title: Fan-out in gene regulatory networks.
abstract: BACKGROUND
  In synthetic biology, gene regulatory circuits are often constructed by combining smaller circuit components. Connections between components are achieved by transcription factors acting on promoters. If the individual components behave as true modules and certain module interface conditions are satisfied, the function of the composite circuits can in principle be predicted.
  
  
  RESULTS
  In this paper, we investigate one of the interface conditions: fan-out. We quantify the fan-out, a concept widely used in electrical engineering, to indicate the maximum number of the downstream inputs that an upstream output transcription factor can regulate. The fan-out is shown to be closely related to retroactivity studied by Del Vecchio, et al. An efficient operational method for measuring the fan-out is proposed and shown to be applied to various types of module interfaces. The fan-out is also shown to be enhanced by self-inhibitory regulation on the output. The potential role of an inhibitory regulation is discussed.
  
  
  CONCLUSIONS
  The proposed estimation method for fan-out not only provides an experimentally efficient way for quantifying the level of modularity in gene regulatory circuits but also helps characterize and design module interfaces, enabling the modular construction of gene circuits.
authors.affiliation: Department of Bioengineering, University of Washington, William H, Foege Building, Box 355061, Seattle, WA 98195-5061, USA. kkim@uw.edu.
authors.forename: Kyung H
authors.initials: KH
authors.lastname: Kim
authors.forename: Herbert M
authors.initials: HM
authors.lastname: Sauro