Colorectal cancer had a low incidence several decades ago. However, it has become a predominant cancer and now accounts for approximately 10% of cancer-related mortality in western countries. The ‘rise’ of colorectal cancer in developed countries can be attributed to the increasingly ageing population, unfavourable modern dietary habits and an increase in risk factors such as smoking, low physical exercise and obesity. New treatments for primary and metastatic colorectal cancer have emerged, providing additional options for patients; these treatments include laparoscopic surgery for primary disease, more-aggressive resection of metastatic disease (such as liver and pulmonary metastases), radiotherapy for rectal cancer and neoadjuvant and palliative chemotherapies. However, these new treatment options have had limited impact on cure rates and long-term survival. For these reasons, and the recognition that colorectal cancer is long preceded by a polypoid precursor, screening programmes have gained momentum. This Primer provides an overview of the current state of art knowledge on the epidemiology and mechanisms of colorectal cancer, as well as on diagnosis and treatment.

RNA interference (RNAi) is a mechanism by which double-stranded RNAs (dsRNAs) suppress specific transcripts in a sequence-dependent manner. dsRNAs are processed by Dicer to 21-24-nucleotide small interfering RNAs (siRNAs) and then incorporated into the argonaute (Ago) proteins. Gene regulation by endogenous siRNAs has been observed only in organisms possessing RNA-dependent RNA polymerase (RdRP). In mammals, where no RdRP activity has been found, biogenesis and function of endogenous siRNAs remain largely unknown. Here we show, using mouse oocytes, that endogenous siRNAs are derived from naturally occurring dsRNAs and have roles in the regulation of gene expression. By means of deep sequencing, we identify a large number of both approximately 25-27-nucleotide Piwi-interacting RNAs (piRNAs) and approximately 21-nucleotide siRNAs corresponding to messenger RNAs or retrotransposons in growing oocytes. piRNAs are bound to Mili and have a role in the regulation of retrotransposons. siRNAs are exclusively mapped to retrotransposons or other genomic regions that produce transcripts capable of forming dsRNA structures. Inverted repeat structures, bidirectional transcription and antisense transcripts from various loci are sources of the dsRNAs. Some precursor transcripts of siRNAs are derived from expressed pseudogenes, indicating that one role of pseudogenes is to adjust the level of the founding source mRNA through RNAi. Loss of Dicer or Ago2 results in decreased levels of siRNAs and increased levels of retrotransposon and protein-coding transcripts complementary to the siRNAs. Thus, the RNAi pathway regulates both protein-coding transcripts and retrotransposons in mouse oocytes. Our results reveal a role for endogenous siRNAs in mammalian oocytes and show that organisms lacking RdRP activity can produce functional endogenous siRNAs from naturally occurring dsRNAs.

Silencing of transposable elements occurs during fetal gametogenesis in males viaArgonaute proteins, also known as PAZ Piwi domain (PPD) proteins, are members of a well-conserved family that is expressed in a variety of organisms, from fission yeasts to humans. The family can be divided into two subfamilies, Piwi and Ago, based on the primary sequence homology and expression pattern of each member. Piwi subfamily members are expressed only in germ lineage cells, whereas members of the Ago subfamily are expressed ubiquitously. The PPD proteins were initially characterized as essential molecules for stem cell selfrenewal and maintenance in Drosophila, Caenorhabditis elegans, and certain plant species (Cox et al. 1998;Moussian et al. 1998), and a member of the family is essential for stem cell function during regeneration in planaria (Reddien et al. 2005). There are three Piwi subfamily genes in the mouse genome: Miwi (mouse piwi), Miwi2,, which are termed Piwil1 (piwi-like homolog 1), Piwil4, and Piwil2, respectively, in the official nomenclature. Although they are ex-