We show here that the hairpin structure within the 3 0 UTR influences the expression of a luciferase reporter gene. Lowering of luciferase mRNA levels for the construct with an intact hairpin structure in contrast with almost unchanged mRNA levels for the mutated structure indicate that the hairpin is in fact an RNA destabilizing element. However, at the protein level, expression for both constructs exceeds the expression of the control. This indicates the presence of at least two regulatory events in which the hairpin structure is involved: (i) mRNA degradation, (ii) enhanced mRNA transport (possibly coupled with mRNA stabilization) and/or enhanced translation. These effects may be the consequence of the competitive binding of trans-acting factors for the binding site within the hairpin. Thus, identification of the hairpin-binding factors is important for assessment of its physiological role.