A sequence for the green fluorescent protein (GFP) has been used to visually determine the expression of genes inserted into C. elegans (lower photograph). The gene for GFP is injected into the ovary of a worm and becomes incorporated into the worm genome. The expression of this transgene produces GFP, which fluoresces green (upper photograph).

[Huaqi Jiang, Rong Guo, and Jo Anne Powell-Coffman. The Caenorhabditis elegans hif-1 gene encodes a bHLH-PAS protein that is required for adaptation to hypoxia. PNAS 98: 7916–7921, 2001. ©2001 National Academy of Sciences, U.S.A.]