The function of a protein can be described at three different levels:

• Molecular/biochemical function: e.g. phosphatase, DNA-binding protein.
• Cellular function: e.g. growth factor signaling pathway, amino acid metabolism, DNA synthesis.
• Biological funciton: e.g. control of bone growth, regulation of flower development.

Molecular and cellular functions are the easiest to standardize because they can be described very precisely (e.g. the reaction catalyzed by an enzyme) but a standard approach for the classification of biological function is more difficult to envisage.
Another complication is that in bacteria and unicellular eukaryotes, the cellular and biological functions of a protein would be combined, whereas in multicellular organisms they could be separated. As an example, the info below shows the possible classifications for the human protein glucokinase.

- Molecular/biochemical function: kinase, substrate glucose
- Cellular functions: glycolysis, glucose metabolism
- Biological function: expressed specifically in pancreatic Beta-cells and hepatocytes, primary regulator of glucose-controlled insulin secretion, loss of function mutations cause diabetes, gain of function mutations can cause hyperinsulinism.