Phosphoprotein

A phosphoprotein is a protein that is posttranslationally modified by the attachment of either a single phosphate group, or a complex molecule such as 5'-phospho-DNA, through a phosphate group. The target amino acid is usually serine, threonine, or tyrosine residues (mostly in eukaryotes), or aspartic acid or histidine residues (mostly in prokaryotes).

The phosphorylation of proteins is a major regulatory mechanism in cells.

Phosphoproteins have been proposed as biomarkers for breast cancer.