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A 2-kb genome fragment cloned from potato cv. Bastoneza closely resembles CONSTANS, the arabidopsis gene for the photoperiodic control of flowering, and its homologs in tomato, cottonwood, and barley. The potato homolog is immediately related to tomato CONSTANS 1; their exons and introns were 94 and 82% identical. Within the most variable middle region of CONSTANS protein and its homologs, asparagine-and glutamine-rich motifs are of particular interest as they are probably involved in transcription regulation.