Relationship between efficiency of ASLV-A Env mediated virus entry and the number of Tva800 or Tva950 receptors needed for entry. The relative levels of receptor positive cells infected (% cells transduced, Y-axis) when titrating virus (μl, X-axis) onto cells previously transduced with different amounts of a vector encoding Tva800 or Tva950. Graph (a) shows the model for Tva950 when 1 receptor is needed, (b)-(d) assume binding of 1-3 Tva800 receptors, respectively. Data from the multiple-receptor model is not shown for Tva950. We note that the one-receptor model is a special case of the multiple-receptor model (the latter will approach the former as gamma tends to infinity). As the fit for the one-receptor model is very good, any results for larger numbers of receptors would be extremely close to those described here.