**2.1 Blood**

272 Biomarker

al., 2009). Low complexity, no known post-processing modifications, simple detection and amplification methods, tissue-specific expression profiles, and sequence conservation between humans and model organisms make extracellular miRNAs ideal candidates for genomic biomarkers to reflect and study various physiopathological conditions of the body. Ideally, the most clinically powerful information would come directly from the tissue of interest. To understand cancer, one must look at malignant cells, much as one must analyze brain tissue to understand the complexities of neuroscience. However, many of these tissues are difficult to access or impossible to reach without potential injury to the patient. Alternative, or "surrogate", tissues can provide a means of assessing the genomic changes in the tissue of interest, without fear of harming the donor. For example, surrogate tissues may contact the tissue of interest and retain sloughed cells, secreted molecules or the contents of dying cells. While these molecular signals may not exactly mirror the tissue of origin, in many cases they are reproducible and can clearly point to underlying biology. Clinical material suitable for biomarker testing can be divided into 2 different types. The first are those that require minimally invasive procedures to obtain. This type includes blood, cerebrospinal fluid, tissue biopsies and so on. Type 2 tissues are those that can be obtained without any invasive means: hair, saliva, tears, epidermal cells, urine, etc. In some cases, acquisition of the material may not be passive. Examples of Type 1 and Type 2 samples are

listed in Table 1.

Dental Plaque Vascular Plaque

**Type 1 Samples Type 2 Samples** 

Ductal Lavage Nipple Aspirate Fluid

Table 1. Example sample types for the development of genomic biomarkers.

The easier it is to provide a sample for biomarker testing, the greater will be the utilization and utility. There is emerging data that many tissues and fluids that have been largely ignored, hold numerous important analytes that can be exploited for biomarker development. Relative ease of acquisition and rich genomic information, make these surrogate tissues ideally suited for the development of new biomarkers. By casting a wider net over the potential sources of biomarkers, we can increase the odds of finding clinically important ones that will make predictive, personalized healthcare a reality (Hood & Friend, 2011). In this review we will provide examples of various surrogate tissues that are being utilized for the development of genomic biomarkers, and highlight important concepts for

Whole Blood Hair Serum Tears Plasma Breast Milk Cerebral Spinal Fluid Vaginal Secretions

Nasal Scrape Semen Sputum (Lavage) Saliva Bone Marrow Urine Skin Punch Feces Buccal Swab Sweat

successful collection and handling of them.
