Z-box of insulin gene

Z is a regulatory sequence for the insulin gene. Z is made of 2 different sequences(Za & Zb) in the insulin gene.
Z behaves as an enhancer for the transcription of insulin gene in fetal and mature primary islet cells. However, it acts as a repressor for all other cells including immortalized beta cells and beta tumor cells. The deletion of the Z sequence from the insulin gene of primary islet cells results in a decreased insulin production while similar deletions in other cells increase insulin production.
The activity of Z is correlated to the concentration of glucose. The activity of Z is high at higher concentrations of glucose.