Preparation of complete gp190/MSP (merozoite surface protein)-1 (I) of Plasmodium, particularly P, falciparum, comprises expressing the corresponding complete gene in a suitable system, preferably a host organism. Also claimed are: (1) a DNA (II) suitable for expressing (I); (2) host cells containing (I) and/or the sequence encoding it; (3) a method for stabilising gene sequences (A) by reducing the AT content; (4) stabilised genes of reduced AT content; (5) vectors containing (II) or (A), and (6) host cells containing such vectors.