Several CuZn-superoxide dismutases (SODs; EC 1.15.1.1) were cloned from hybrid aspen (Populus tremula L. x tremuloides Michx.). Two of the cloned genes encode representatives of a novel type of CuZn-SOD and we named it HipI-SOD because of its high isoelectric point (> or =9). The SODs were cloned by screening a cDNA library with a probe based on a Scots pine (Pinus sylvestris L.) CuZn-SOD that is predominantly located extracellularly. The expression pattern of HipI-SOD was examined using a Northern blot technique and compared with the expression patterns of cytosolic and chloroplastic SODs. Distinct expression patterns were observed for the three types of CuZn-SOD, with HipI-SODs showing strong expression in apical tissues. Southern blots as well as protein analysis suggest that these novel HipI-SODs belong to a small gene family, one member of which might be monomeric.