The National Cancer Institute defines genomics as the study of the complete set of DNA (including all of its genes) in a person or other organism. The genome contains all the information needed for an individual to develop and grow. Analyzing the genome may help researchers understand how genes interact with each other and the environment and how certain diseases, such as cancer, diabetes, or heart disease develop. This may lead to new ways to diagnose, treat, and prevent disease [15]. Genetic alterations have been identified as major players in tumourigenesis. Therefore, genomics has gained attention as a tool to identify genetic markers that can lead to better diagnosis and prognosis and at the same time, allow researchers to improve the understanding of cancer. Apart from gene mutations and single nucleotide polymorphisms (SNP), the epigenetic signature has also proven useful to establish a more personalised diagnosis [16].

The development of high-throughput methods for genome and gene expression studies has increased the amount of information available. These data are deposited in international public repositories and can be studied by other research groups. NCBI Gene Expression Omnibus (GEO) is the most important database repository of high-throughput gene expression data and hybridization arrays, chips, and microarrays [17]. The Cancer Genome Atlas (TCGA) of the National Cancer Institute (NCI) is another relevant database in oncology. TCGA is a project to classify the genetic mutations that cause cancer, using genome sequencing and integrating bioinformatics tools to analyse this information [18].

Finally, the use of metagenomics, which evaluates the microbiome genes, holds special promise for CRC. Metagenomics has shown the potential to identify differences between control and CRC-associated microbiomes and eventually describe new CRC biomarkers [19].

Transcriptomics is the study of all RNA molecules in a cell and could give more information about how genes are turned on and off in different cell types and how this can contribute to cancer [20]. Differential gene expression comparison studies have emerged as a prospective approach to detecting promising biomarkers of enormous clinical value. This type of study is fuelled by and analyses the data deposited in the TGCA and GEO databases [21].

Proteomics is the study of the structure and function of proteins, including how they work and interact with each other [22]. In the search for new CRC biomarkers, proteomics studies are focused on differential protein expression between normal and cancer cells or the detection of different proteomic profiles in corporal fluids. Some of the most useful techniques for the identification of protein biomarkers in cancer are two-dimensional gel electrophoresis coupled with liquid chromatography/mass spectrometry (2-DE-MS), two-dimensional difference gel electrophoresis (2D-DIGE), or liquid chromatography–mass spectrometry (LC-MS) [23]. Multiplexed quantitative proteomic assays are capable of measuring changes in proteins and their interacting partners, isoforms, and post-translational modifications [23].

Metabolomics is the study of metabolites in cells and tissues, which can be measured in different body fluids. The presence of a tumour can alter the whole individual’s metabolism, and the use of some fuels can be modified to meet the energy demands of the tumour. Furthermore, the tumour metabolism may change as the tumour progresses. Considering that the dysregulation of metabolism is one of the hallmarks of cancer, this omics could open a new way to study cancer [24].

Glycomics studies the structure and function of glycans, N- and O- linked glycoproteins, glycolipids, and proteoglycans ^[25][26][25,26]. The most common alterations in lipid and protein glycosylation are an increase in the branching of N-glycans, high density of O-glycans, incomplete glycans synthesis, neosynthesis, and sialylation and fucosylation increase [27]. Glycans characterization can be done by a large number of techniques, such as microarrays, flow cytometry, enzyme-linked immunosorbent assay, mass spectrometry, and chromatographic techniques [27].

Volatolomics is the study of volatile organic compounds that have high vapor pressure. This is a non-invasive, fast, and potentially inexpensive way of analysing the human body chemistry for monitoring of diseases such as cancer [28]. The volatilome, volatile organic compounds (VOC) profile, is being used in the detection of CRC. Alterations in the metabolism of cancer cells can be reflected in a characteristic profile of VOCs, as these compounds are produced in metabolic processes such as inflammation, cancer metabolic alterations, and necrosis processes ^{[29][30][31][32][33]}[29,30,31,32,33]. Cancer-associated VOCs are directly excreted from the affected organ or tissue to stool or blood. Thus, the VOCs are exhaled in breath, excreted in urine, or released from the skin ^[34][35][36][34,35,36]. However, the VOCs interactions with the microbiota may affect the volatilome of stool [29]. The most used techniques in volatolomics are gas chromatography with mass spectrometry (GC–MS), which enables the separation and quantification of individual VOCs; proton transfer reaction—mass spectrometry (PTR-MS), for simultaneous real-time monitoring of VOCs without sample preparation; and eNose, which allows the analysis of a specific VOC pattern in real-time. The latter is a low cost, easy-to-use equipment that can detect cancer at an early stage and can differentiate between cancer and healthy subjects ^[29][33][37][29,33,37].

Several studies have demonstrated the potential of the exhaled volatilome for CRC diagnosis and screening due to its sensitivity and specificity. However, further studies and standardization of collection and analysis methods for volatilome detection and its application to CRC diagnosis are needed ^{[31][35][36][37][38][39]}[31,35,36,37,38,39].