Hydroxy-PEG1-acid sodium is a PEG linker containing both a hydroxyl group and a terminal carboxylic acid (in sodium salt form). The free acid form is unstable because the OH group reacts with PEG-COOH to form polymers. The sodium salt version remains stable during storage and transport. The hydrophilic PEG spacer enhances solubility in aqueous media. The hydroxyl group can be further derivatized or replaced by other reactive functional groups.