The present invention relates to an isolated nucleic acid molecule comprising a sequence of nucleotides encoding a .beta.-ketoacyl ACP synthase II from a plant, or a variant, derivative, homolog or analog of said .beta.-ketoacyl ACP synthase II. The invention also relates to an isolated .beta.-ketoacyl ACP synthase II enzyme from a plant. The polynucleotide sequences of the present invention may be used inter alia for the generation of cells or plants comprising such cells which have a modified phenotype and/or which exhibit a desired characteristic.