Despite these variations, some overall trends are commonly seen. The RBE is generally independent of LET for any LET less than 10 keV/µm, so a low LET is normally chosen as the reference condition where RBE is set to unity. Above 10 keV/µm, some systems show a decline in RBE with increasing LET, while others show an initial increase to a peak before declining. Mammalian cells usually experience a peak RBE for LET's around 100 keV/µm. These are very rough numbers; for example, one set of experiments found a peak at 30 keV/µm.