Mannuronan C5-epimerases (ManC5-Es) catalyse in brown algae the remodelling of alginate, a major cell wall component which is involved in many biological functions in these organisms. ManC5-Es are present as large multigenic families in brown algae, likely indicating functional specificities and specialisations. ManC5-Es control the distribution pattern of (1-4) linked b-D-mannuronic acid (M) and a-L-guluronic acid (G) residues in alginates, giving rise to widely different polysaccharide compositions and sequences, depending on tissue, season, age or algal species. As such they are also a source of powerful new tools for the biotechnological and enzymatic processing of alginates, to match the growing interest for food hydrocolloids and in biomedical and nanotechnological applications. We report here the first heterologous production of a ManC5-E of brown algal origin that is successfully refolded in an active form. The activity was measured by 1H-NMR and by an indirect enzymatic assay using a known bacterial alginate lyase. The transcript expression as a function of the developmental program of the brown alga Ectocarpus, together with the bioinformatic analyses of the corresponding gene context of this multigenic family, are also presented.