The metabolic pathway for synthesis of the amino acid arginine in Neurospora was established by providing mixes of supplements to seven different auxotrophic arginine mutants.

• Mutants arg 4–7 grew if supplied with either citrulline, ornithine, or arginine.

• Mutants arg 2 and arg 3 grew if supplied with citrulline or arginine.

• Mutant arg 1 grew only if arginine were supplied; neither citrulline nor ornithine enabled growth.