We have isolated three major cDNA fragments of protein phosphatase inhibitor-1 from human brain and liver by RT-PCR. The 536 bp fragment encoded the wild-type of inhibitor-1 while two other fragments were alternative splice products of the inhibitor-1 gene, which was confirmed by partial genomic DNA sequencing. The 380 bp fragment encoded an in-frame 51-residue-deleted inhibitor-1, named inhibitor-1alpha, and the deletion occurred from residue 84 to 134 of inhibitor-1. The 316 bp fragment termed inhibitor-1,6 was derived from an internal deletion of 536 bp fragment. This deletion resulted in an out of frame shift, allowing the 316 bp fragment that encoded the partial sequence of inhibitor-1. Based on the reported mRNA sequence of inhibitor-1 and evidence from our RTPCR, we suggested that inhibitor-1beta consisted of 132 amino acids of which the N-terminal 61 amino acid sequences were identical to inhibitor-1 while the sequence after residue-61 was markedly different. (C) 2002 Elsevier Science (USA).