The RSA provides an initial condition for (10a) under the variable transformation t → t -t c1 . The mathematical expression (13b) is the MM equation, and the system (13a)-(13b) governs the dynamics of the substrate s 1 and complex c 1 of the non-observable reaction under the QSS and RSA. The explicit closed-form solution of (13b), with the initial condition , is known as the Schnell-Mendoza equation , and is written in terms of the Lambert-W function: