Is formaldehyde a crosslinking agent?

Yes, formaldehyde is a powerful cross-linking agent. 

Crosslinking is a laboratory process that involves chemically joining two or more molecules by a covalent bond. A crosslinking agent is a molecule that has two or more reactive ends that are capable of attaching chemically to specific functional groups such as sulfhydryls and primary amines on proteins and other molecules. Formaldehyde’s small size makes it an efficient cross-linking agent. Its small size allows formaldehyde to permeate cell walls and membranes easily where it forms covalent bonds between reactive molecules including proteins and DNA. Formaldehyde is routinely used to cross-link proteins in cells and tissue to study binding of specific proteins to DNA elements in undamaged cells. Formaldehyde cross-linking is an integral component of several laboratory processes including immune-histological analysis of protein expression and chromatin immunoprecipitation (ChIP) of protein-DNA complexes found in nuclei among others.