This is hampered by differences between rat and human P2X7 receptor, often resulting in a lower affinity of compounds for the rodent receptors compared with the human receptor, complicating preclinical evaluation of tracers targeting P2X7R. However, using [3H]A-740003, a non-brain penetrant P2X7R antagonist with similar affinities for rat (18 nM) and human P2X7R (40 nM)12,13, increased P2X7R expression was recently shown to be highest at the peak of the disease in a rat model of MS in vitro20. This evidence concerns the gene P2RX7 and myeloid sarcoma.