Provided are isolated nucleic acid molecules which comprise VRN1 nucleotide sequences, which encode a polypeptide which is capable of specifically altering the vernalisation response of a plant into which the nucleic acid is introduced and expressed. Examples include cDNA and gDNA sequences (see e.g., Annex I). Also provided are variant molecules which may be derivatives or homologues (e.g., alleles, or paralogues such as RTV1), plus also complementary molecules. Corresponding polypeptides form a further part of the invention. The invention also provides methods and materials for preparing and using these molecules, e.g., in the production of plants having modified vernalisation characteristics. Also provided are methods for influencing and assessing the vernalisation phenotype of a plant.