CD38 and plasma cell myeloma: Given the general overexpression of CD38 in myeloma, we assume that δN=10δP (Sect B in S2 Text suggests limited sensitivity to the exact ratio), and that fitness is significantly lower in N. It has been shown that CD38 is a growth and survival factor in a lung cancer model system [5], and so it is reasonable to assume that N has both lower proliferation and higher death rate than P. We also allow for both direct drug-induced loss of CD38 expression (δPu), and an off-target effect modelled by drug-induced death of healthy cells (μAu).