where T and I are now the number of target and infectious cells, N is the total number of cells, V is the homogeneous virus concentration and all parameters have the same meaning as in the PDE model above. These equations can be derived from Equation (1) by assuming spatial homogeneity and integrating over space. An expression for the exponential growth rate, l g , of viral titer in the multiple-cycle yield assay can be derived from this system by assuming that in the early phases of an infection (well before the viral titer peak): the number of target cells is approximately constant T(t)~T 0 &N; there is an exponential growth of infectious cells I(t)~I 0 e lgt and virus V (t)~e lgt with common rate l g ; and infectious cell death can be neglected (t I ??). Substituting these expressions into (2) yields a transcendental equation for the viral titer growth rate: