To model this scenario, we introduce the PTHrP concentration  as a new state-variable: once produced by cancer cells (rate ), PTHrP diffuses across the tissue (rate ) and is degraded by proteases (rate ),(5)While diffusing across the tissue, PTHrP induces the expression of RANKL by osteoblastic cells in the bone tissue, and we describe this by adding a source term () to the RANKL equation (3). The gene discussed is TNFSF11; the disease is cancer.