**2. Personalized medicine and pharmacogenetics**

The term "Personalized Medicine" was not long ago some "scifi" concept, just express‐ ing the best wishes of the scientific community with an aim of adjusting the pharmaco‐ therapy as best as possible to each single patient. However, in the last years we have seen real advances in this area that have brought to the real clinical practice in most of the "first world" countries, a set of new analysis under the same principle: offering an individualized therapy to each different patient.

In order to understand this new approach in medicine and put it into practice, we necessarily have to take genetics in consideration, and particularly, we have to pay attention to the individual differences that make each patient respond in a different way to a given pharma‐ cological treatment. Here, we arrive to the concepts of Pharmacogenetics and Pharmacoge‐ nomics, that can be heard in more and more places each day. They are, and for sure will be, components to be considered in the medical practice. We can define them in many ways, and traditionally they have been employed interchangeably although there are differences between them. They are different but complementary disciplines. The European Medicines Agency, EMA, takes their definitions from The International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH), this is a project with regulatory authorities from Europe, Japan and USA, together with experts from pharmaceut‐ icals that discusses technical and scientific aspects about the products registries. One of its aims is to reach a better harmonization in the interpretation and application of technical guides and requirements for the registries. ICH defines Pharmacogenomics as the study of variations in DNA and RNA characteristics regarding the response to drugs and it defines Pharmacoge‐ netics as a subset inside pharmacogenomics, that studies the variations in the DNA sequence regarding the response to drugs [1-5].

There is another term that is also frequently found: biomarker, genetic or genomic biomarker, whose definition is "a measurable characteristic of DNA and/or RNA which is an indicator of a biological process that can be normal, pathogenic and/or a response to a therapeutic (or other kind) intervention. A genomic biomarker could be, for instance, the measuremente of a gene expression or of its regulation. It can consist in one or more DNA and/or RNA characteristics, as for instance, in DNA, its single nucleotide polymorphisms (SNPs); the variability in the repetition of short sequences; the haplotypes; DNA modifications as methilation; the deletions or insertions of a single nucleotide; the copy number variation; or the cytogenetic rearange‐ ments as translocations, duplications, deletions or inversions. Regarding RNA, it could be a particular trait of its sequence; the levels of expression; the processing (as splicing and editing); or the levels of microRNAs. A deeper explanation of some of these terms will be done in the next paragraph.

The aim of pharmacogenomics is to identify the most important genetic elements in the instauration and/or evolution of a pathological process in order to create new strategies for drug evaluation and optimization of the drug development process. These are usually high throughput studies, regarding the simple number and statistical signification but also very exigent with the study subject: the final goal is finding correlation with the disease at a genomic level, not with one single nucleotide but with genes or groups of related genes instead. On the other side, pharmacogenetics studies the influence of genetic factors on the activity of a drug, making attention in concrete changes inside a gene that somehow has already been postulated as a candidate gene, by previous knowledge or by pharmacogenomic studies. Subject of pharmacogenetic studies are especially, the variants in genes related with transport and metabolism of drugs, with the aim that specific drugs can be given to specific groups of genetically defined (or "stratified") patients [6, 7].

To summarize, pharmacogenetics has to be considered as one of the mainstays of personalized medicine, which will let us correlate good or bad response to a drug in a specific population with genetic aspects. It will also let us know which drugs will offer greater therapeutic benefit or lower risk of adverse reactions development for a given population.
