Objective: Phorbol-esters may function as inducers of cellular maturation/ differentiation processes. hASH1 (human achaete-scute homologue- 1, syn.: ASCL1) belongs to the family of bHLH (basic helix-loop-helix) transcription factors and has an important role in the control of neuronal development. The aim of this study is to investigate the influence of phorbol-ester on hASH1 gene expression rate. Methods: Human neuroblastoma-derived Kelly cells were treated with phorbol 12-myristate 13-acetate (PMA), and dose-dependent effects on hASH1 expression were assessed at different time intervals. Protein levels of hASH1 were estimated by Western blotting and mRNA levels were determined by qRT- PCR. The relevance of potential cis-regulatory control elements was explored in reporter gene assays. Results: PMA treatment caused a fast (to 48% +/­13% compared to control within 3 h) and strong (to 11% +/­6% after 48 h) suppression of the hASH1 gene expression rate. This inhibition was observed at the mRNA as well as at the protein level. Reporter gene assays with the use of constructs, in which the luciferase activity was controlled by the hASH1 mRNA untranslated regions (UTRs), revealed an UTR-mediated suppression after 12 h of PMA treatment. The hASH1 promoter activity was decreased only after 48 h of PMA administration. The Protein kinase C (PKC) inhibitor staurosporine reduced the effect of PMA only moderately, suggesting a PKC- independent mechanism of action. Conclusion: Treatment of neuroblastoma cells with phorbol ester inhibits hASH1 synthesis, a factor important for neuronal enddifferentiation. Suppression of hASH1 synthesis is attributed to different mechanisms, which involve UTR mediated, posttranscriptional control and decreased promoter activity. It is suggested that inhibition of hASH1 favours malignant neuroblastoma cell growth.