Hydroxy-PEG4-acid sodium is a PEG linker featuring both a hydroxyl group and a terminal carboxylic acid (in its sodium salt form). The free acid form is unstable due to the reaction between OH and PEG-COOH groups, resulting in polymer formation. However, the sodium salt form remains stable during storage and transport. The terminal carboxylic acid can react with primary amine groups in the presence of activators such as EDC or HATU to form stable amide bonds. The hydrophilic PEG spacer enhances solubility in aqueous media. The hydroxyl group can be further derivatized or substituted with other reactive functional groups.