Protein is found in every living cell.

Conditionally essential amino acids are those only required during certain life stages.  Examples include glutamine during recovery from traumatic injuries, or cysteine and arginine during neonatal prematurity.

There are 20 total amino acids needed to make all the proteins necessary for proper human health.  We can synthesize 11 of them if our diet is adequate in overall nutrients and calories.  We must consume 9 essential amino acids through our diet.  Dietary variety is the best way to ensure adequate amino acid and protein intake.