"The pancreas has always been difficult to study," Dr. Liddle said.
But the use of visual cues has been difficult to study, because if a bird flies over a landmark and doesn't change its course, it's impossible to know whether the bird has not perceived the cue or is ignoring it.
The NMSZ has been difficult to study.
The drug's subtle effect has been difficult to study in cultured cells and in laboratory animals.
The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools.
This phenomenon has been difficult to study because of a lack of quantitative and high-throughput cellular tools.
The activation thresholds and atomic mechanisms that dictate the thermo-kinetic properties of grain boundaries have been difficult to study due to complex and highly variable GB structure.
But that phenomenon has been difficult to study in the lab, because the grains are always pulled down by gravity, limiting the interactions between particles.
It has been difficult to study their localization and function in vivo due to the lack of ligands that exclusively act on one subtype of the receptor.
The fusion glycoproteins have been difficult to study because they are present in a labile, metastable form at the surface of infectious virions.
The podocyte is a key cell in the glomerulus in health and disease: until recently, human podocytes have been difficult to study in vitro.
