Genes involved in photoperiod flowering are diurnally regulated (2–4, 7, 33–37). We therefore examined the diurnal expression patterns of ZmCCT9 in mature leaves of NIL(maize) and NIL(teosinte) under artificial LD and SD conditions. Under LD conditions, ZmCCT9 transcript levels in NIL(teosinte) showed strong diurnal oscillation, with the highest expression 3 h after dawn; in contrast, the amplitude of ZmCCT9 expression in NIL(maize) was significantly attenuated (Fig. 3E). Under SD conditions, ZmCCT9 in NIL(maize) and NIL(teosinte) showed similar weak diurnal expression patterns (Fig. 3E). These results suggest that ZmCCT9 is diurnally regulated and that the maize allele exhibits a much reduced photoperiod sensitivity relative to the teosinte allele, likely due to the repressive effect of the Harbinger-like element in the 2.4-kb causative region for qDTA9.