Editing Chronobiology (section)

===Diversity in M1 cells===
Hattar and his co-workers discovered that, even among the subtypes of ipRGC, there can be designated sets that differentially control circadian versus pupillary behavior. In experiments with M1 ipRGCs, they discovered that the transcription factor [[POU4F2|Brn3b]] is expressed by M1 ipRGCs that target the OPN, but not by ones that target the SCN. Using this knowledge, they designed an experiment to cross Melanopsin-[[Cre-Lox recombination|Cre]] mice with mice that conditionally expressed a toxin from the Brn3b locus. This allowed them to selectively ablate only the OPN projecting M1 ipRGCS, resulting in a loss of pupil reflexes. However, this did not impair circadian photo entrainment. This demonstrated that the M1 ipRGC consist of molecularly distinct subpopulations that innervate different brain regions and execute specific light-induced functions.<ref name=":1"/> This isolation of a 'labeled line' consisting of differing molecular and functional properties in a highly specific ipRGC subtype was an important first for the field. It also underscored the extent to which molecular signatures can be used to distinguish between RGC populations that would otherwise appear the same, which in turn facilitates further investigation into their specific contributions to visual processing.<ref name=":1"/>