**2.2 Cerebrospinal fluid**

Cerebrospinal fluid (CSF) is a cell-free, colorless liquid that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord. It is usually obtained through lumbar puncture. CSF has been rediscovered in the post-genomic era, as a great source of potential protein biomarkers for various diseases as it bathes the brain and other neurological tissues. Analysis of CSF allows rapid screening, low sample consumption, and accurate protein identification by proteomic technology (Guerreiro et al., 2006; Zheng et al., 2003). Brain proteins in CSF are also important for diagnosis of noninflammatory CNS diseases. Examples of conditions in which these proteins are diagnostically relevant include degenerative diseases (Otto et al., 1997; Ranganathan et al., 2005), tumors (Zheng et al., 2003), hypoxias and brain infarction (Schaarschmidt et al., 1994).

Advancements in nucleic acid (NA) amplification techniques have transformed the diagnosis of bacterial and viral infections of the central nervous system. Because of their enhanced sensitivity, these methods enable detection of very low amounts of pathogenic genomes in cerebrospinal fluid. Diagnosis of several viral CNS infections, such as herpes encephalitis, enterovirus meningitis and other viral infections occurring in human immunodeficiency virus-infected persons are currently performed using cerebrospinal fluid (Cinque, Bossolasco, & Lundkvist, 2003). MicroRNAs are also becoming an important analyte in CSF for the identification of neurological disease (Baraniskin et al., 2011; Cogswell et al., 2008; De Smaele et al., 2010). For example, miRNAs isolated from the frozen cerebrospinal fluid of Alzheimer disease-affected (AD) and non-affected patients showed distinctly different expression profiles (Cogswell et al., 2008). Notably miRNAs linked to immune cell functions including innate immunity and T cell activation and differentiation were up-regulated in AD.

Combining mRNA studies with protein expression analysis may provide a more global picture of the biological processes associated with CNS disorders. Information gathered could lead to the development of select biological indices (biomarkers) for guiding CNS diagnosis and therapy.

#### **2.3 Saliva**

Saliva is an easily obtainable tissue that has been used in forensics for decades (Sweet et al., 1997). However, new molecular profiling kits for voluntary saliva collection have made saliva an increasingly useful clinical biomarker tissue. The collection process is noninvasive, and can even be collected at home or in isolated locations using some of the newer collection kits (Oragene or Norgen products). This ease of collection results in higher compliance by the patients. As is often the case in biological samples, the difference in yield is usually a donor dependent value (van Schie & Wilson, 1997). It is possible that saliva samples could replace blood samples for DNA studies. A study in Australia and New Zealand compared 10 matched pairs of blood and saliva, as well as nearly 2000 samples of either blood (Australia) or saliva (New Zealand; Oragene collection system) for genotyping. This study was larger than the van Schie & Wilson study, but corroborated that there is a donor dependency to DNA yield. Because of the larger sample number, they saw more sample variance. However, they also concluded that variance had more to do with collection, processing and donor variability than variance due to tissue type (Bahlo et al., 2010). The collection and processing methods can all eventually be controlled. In most cases there was enough mass from 1 ml of saliva sample to yield at least 4ug of DNA, which is enough DNA for most molecular biology assays.
