−1 programmed ribosomal frameshifting (−1 PRF) is a translational recoding mechanism used by many viral and cellular mRNAs. −1 PRF occurs at a heptanucleotide slippery sequence and is stimulated by a downstream RNA structure, most often in the form of a pseudoknot. The utilization of −1 PRF to produce proteins encoded by the −1 reading frame is wide-spread in RNA viruses, but relatively rare in cellular mRNAs. In human, only three such cases of −1 PRF events have been reported, all involving retroviral-like genes and protein products. To evaluate the extent of −1 PRF utilization in the human transcriptome, we have developed a computational scheme for identifying putative pseudoknot-dependent −1 PRF events and applied the method to a collection of 43,191 human mRNAs in the NCBI RefSeq database. In addition to the three reported cases, our study identified more than two dozen putative −1 PRF cases. The genes involved in these cases are genuine cellular genes without a viral origin. Moreover, in more than half of these cases, the frameshift site locates far upstream (>250 nt) from the stop codon of the 0 reading frame, which is nonviral-like. Using dual luciferase assays in HEK293T cells, we confirmed that the −1 PRF signals in the mRNAs of CDK5R2 and SEMA6C are functional in inducing efficient frameshifting. Our findings have significant implications in expanding the repertoire of the −1 PRF phenomenon and the protein-coding capacity of the human transcriptome.