Sp1 represses the D4ECR1 expression in dissociated cultures of neonate rat frontal cortex. Transcriptional activities of D4ECR1 and control plasmid (pGL3p) were equalled to 100%. Both D4ECR1 and pGL3p plasmids were co-transfected with 2 concentrations of Sp1 expression vector (0.5 and 1 μg). Sp1 significantly down regulated transcriptional activity supported by D4ECR1 (Student's T-test p ≤ 0.01 = **; n = 9, 2 day old neonates). The effect of Sp1 over-expression was expressed as the percentage repression on basal transcriptional activity of D4ECR1 and pGL3p.