MicroRNAs (miRNAs) are endogenous small non-protein coding RNAs that play important regulatory roles in animals and plants by binding to target transcripts for cleavage or translational repression. Despite increasing number of genes has been predicted to be miRNA targets by bioinformatics analysis and luciferase-based reporter assay in vitro (RA-In Vitro), few of them have been experimentally validated in physiological context. Using transient reporter assay in vivo (TRA-In Vivo), we identified hydroxymethylbilane synthase b (hmbsb) and Kruppel-like transcription factor d (klfd) as potential target gene for miR-451 and miR-144, respectively. Although hmbsb, miR-451, klfd and miR-144 are all co-expressed in the developing erythroid progenitors during zebrafish erythropoiesis, only klfd can be validated as a bona fide physiological target of miR-144 using a transgene-based physiological reporter assay in vivo (PRA-In Vivo). Thus, failure to verify hmbsb as miR-451 target in physiological context raises a crucial question as to how to determine bona fide target of miRNA. The results address the importance of using multiple approaches combined with Western blot analysis to validate the physiological target of a given miRNA. (C) 2009 Elsevier Inc. All rights reserved.