MicroRNAs play important roles in animal development, cell differentiation, and metabolism and have been implicated in human cancer. The let-7 microRNA controls the timing of cell cycle exit and terminal differentiation in Caenorhabditis elegans and is poorly expressed or deleted in human lung tumors. Here, we show that let-7 is highly expressed in normal lung tissue, and that inhibiting let-7 function leads to increased cell division in A549 lung cancer cells. Overexpression of let-7 in cancer cell lines alters cell cycle progression and reduces cell division, providing evidence that let-7 functions as a tumor suppressor in lung cells. let-7 was previously shown to regulate the expression of the RAS lung cancer oncogenes, and our work now shows that multiple genes involved in cell cycle and cell division functions are also directly or indirectly repressed by let-7. This work reveals the let-7 microRNA to be a master regulator of cell proliferation pathways. [Cancer Res 2007;67(16):7713-22]

PrimerBank (http://pga.mgh.harvard.edu/primerbank/) is a public resource for the retrieval of human and mouse primer pairs for gene expression analysis by PCR and Quantitative PCR (QPCR). A total of 306 800 primers covering most known human and mouse genes can be accessed from the PrimerBank database, together with information on these primers such as Tm, location on the transcript and amplicon size. For each gene, at least one primer pair has been designed and in many cases alternative primer pairs exist. Primers have been designed to work under the same PCR conditions, thus facilitating high-throughput QPCR. There are several ways to search for primers for the gene(s) of interest, such as by: GenBank accession number, NCBI protein accession number, NCBI gene ID, PrimerBank ID, NCBI gene symbol or gene description (keyword). In all, 26 855 primer pairs covering most known mouse genes have been experimentally validated by QPCR, agarose gel analysis, sequencing and BLAST, and all validation data can be freely accessed from the PrimerBank web site.

Optimization of primer sequences for polymerase chain reaction (PCR) and quantitative PCR (qPCR) and reaction conditions remains an experimental challenge. We have developed a resource, PrimerBank, which contains primers that can be used for PCR and qPCR under stringent and allele-invariant amplification conditions. A distinguishing feature of PrimerBank is the experimental validation of primer pairs covering most known mouse genes. Here, we describe a major update of PrimerBank that includes the design of new primers covering 17 076 and 18 086 genes for the human and mouse species, respectively. As a result of this update, PrimerBank contains 497 156 primers (an increase of 62% from the previous version) that cover 36 928 human and mouse genes, corresponding to around 94% of all known protein-coding gene sequences. An updated algorithm based on our previous approach was used to design new primers using current genomic information available from the National Center for Biotechnology Information (NCBI). PrimerBank primers work under uniform PCR conditions, and can be used for high-throughput or genome-wide qPCR. Because of their broader linear dynamic range and greater sensitivity, qPCR approaches are used to reanalyze changes in expression suggested by exploratory technologies such as microarrays and RNA-Seq. The primers and all experimental validation data can be freely accessed from the PrimerBank website, http://pga.mgh.harvard.edu/primerbank/.