Using GFP markers to aid cell-cycle studies. (Top) Schematic drawing of a late L1 larva with intestinal and ventral cord cells indicated. The cell lineage for an intestinal nucleus (In) and ventral cord precursor cell (P8) is drawn to the right. (Bottom) Nomarski DIC and GFP fluorescence image from a late L1 larva carrying a transgene expressing GFP under control of ribonucleotide reductase regulatory sequences (P
::GFP). Imaged area corresponds approximately to the boxed area in the schematic drawing. GFP expression correlates with progression through S-phase, and can thus be used to distinguish a G1 arrest from a later arrest. In this image, P
::GFP is expressed in the descendants of P8.