HIC-1 (hypermethylated in cancer 1), a BTB/POZ transcriptional repressor, was isolated as a candidate tumor suppressor gene located at 17p13.3, a region hypermethylated or subject to allelic loss in many human cancers and in the Miller-Dieker syndrome. The human HIC-1 gene is composed of two exons, a short 5'-untranslated exon and a large second coding exon. Recently, two murine HIC-1 isoforms generated by alternative splicing have been described. To determine whether such isoforms also exist in human, we have further analyzed the human HIC-1 locus. Here, we describe and extensively characterize a novel alternative noncoding upstream exon, exon 1b, associated with a major GC-rich promoter. We demonstrate using functional assays that the murine exon 1b previously described as coding from computer analyses of genomic sequences is in fact a noncoding exon highly homologous to its human counterpart. In addition, we report that the human untranslated exon is presumably a coding exon, renamed exon 1a, both in mice and humans. Both types of transcripts are detected in various normal human tissues with a predominance for exon 1b containing transcripts and are up-regulated by TP53, confirming that HIC-1 is a TP53 target gene. Thus, HIC-1 function in the cell is controlled by a complex interplay of transcriptional and translational regulation, which could be differently affected in many human cancers.