Analysing the relationships between potential biomarkers such as resting and exercise-induced levels of neurotrophin (BDNF) and cytokine (IL-6) and functional measures (walking speed [24,25,26,27], fatigue [28,29], and aerobic fitness (maximal oxygen consumption (V·O2) [30,31,32]) may help determine whether the neurotrophin BDNF and cytokine IL-6 are appropriate rehabilitative targets and surrogate markers of recovery and neuroplasticity in progressive MS [33,34,35,36]. The gene discussed is BDNF; the disease is myeloid sarcoma.