Figure 2. Identification of EGL-13, a factor that specifies both O2- and CO2-sensing neurons. (A) Schematic presentation of egl-13 genetic lesions showing the resultant effects on the EGL-13 protein. (B) In C. elegans, the O2/CO2-sensing neural system comprises of six neurons called BAGL/R, URXL/R, AQR, and PQR (only single BAG and URX cells are shown). This neural system senses and transmits information regarding O2/CO2 status in the external environment and in the body fluid to the nerve ring. (C) Wild-type animals have six functional O2/CO2-sensing (top picture). egl-13 mutant animals (bottom picture) fail to correctly express an O2/CO2-sensing cell fate marker and are unable to sense O2 and CO2.