The PAS domain is a protein domain

Protein domain

A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...

 contained in many signaling proteins where it functions as a signal sensor. PAS domains are found in a large number of organisms from bacteria to humans. The PAS domain was named after the three proteins in which it occurs:

• Per – period circadian protein
  Period (gene)
  Period is a gene located on the X chromosome of Drosophila melanogaster. Oscillations in levels of both per transcript and its corresponding protein PER have a period of approximately 24 hours and together play a central role in the molecular mechanism of the Drosophila biological clock driving...
  
  
• Arnt – aryl hydrocarbon receptor nuclear translocator protein
  Aryl hydrocarbon receptor nuclear translocator
  The ARNT gene encodes the aryl hydrocarbon receptor nuclear translocator protein that forms a complex with ligand-bound aryl hydrocarbon receptor , and is required for receptor function. The encoded protein has also been identified as the beta subunit of a heterodimeric transcription factor,...
  
  
• Sim – single-minded protein
  SIM1
  Single-minded homolog 1 also known as class E basic helix-loop-helix protein 14 is a protein that in humans is encoded by the SIM1 gene.- Function :...
  
  

Many PAS-domain proteins detect their signal by way of an associated cofactor such as heme

Heme

A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...

. Proteins that contain a PAS domain include Hypoxia-inducible factors.