  How cells reliably infer information about their environment is a
fundamentally important question. While sensing and signaling generally start
with cell-surface receptors, the degree of accuracy with which a cell can
measure external ligand concentration with even the simplest device - a single
receptor - is surprisingly hard to pin down. Recent studies provide conflicting
results for the fundamental physical limits. Comparison is made difficult as
different studies either suggest different readout mechanisms of the
ligand-receptor occupancy, or differ on how ligand diffusion is implemented.
Here we critically analyse these studies and present a unifying perspective on
the limits of sensing, with wide-ranging biological implications.
