These are similar developmental processes to those affected by Drifter , a POU homeodomain transcription factor which could be downstream of breathless. The ligand is Branchless , and its developmentally regulated distribution is responsible for the spatially restricted activation of Breathless. Function of the extracellular domain of Breathless can be assessed by substituting a Torso sequence for that of the FGF-receptor sequence. Thus the all purpose RAS-RAF pathway is used for each of these receptors. These cells form a completely different chemical milieu when compared with retinal cells, follicle cells and terminal cells, all of which have receptors that use the RAS-RAF pathway.
Some cell functions are common to both tracheal cells and midline glia.
Cell migration is regulated by Breathless in both types of cells. Use of a heat inducible dominant negative Breathless protein, lacking a functional tyrosine kinase, reveals that breathless is required at the initiation of tracheal cell migration. Distinct subsets of tracheal cells designated as terminal cells extend long processes toward target tissues. Blind-ended tubes called tracheoles are formed connecting to the main tracheal branches. Tracheoles insure the proper supply of air to growing larval tissues.