A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation). Phosphorylation usually results in a functional change of the target protein (substrate) by changing enzyme activity, cellular location, or association with other proteins. The human genome contains about 560 protein kinase genes and they constitute about 2% of all human genes. Up to 30% of all human proteins may be modified by kinase activity[citation needed], and kinases are known to regulate the majority of cellular pathways, especially those involved in signal transduction. Protein kinases are also found in bacteria and plants, and include the pseudokinase sub-family, which exhibit unusual features  including atypical nucleotide binding and weak, or no, catalytic activity  and are part of a much larger pseudoenzyme group of 'degraded' enzyme relatives that are found throughout life, where they take an active participation in mechanistic cellular signaling.