BDNF mRNA in human blood normalized to GAPDH and β 2M. Median BDNF mRNA levels in the blood of eight control participants from the Institute of Neurology, MRC Prion Unit, and University College London (UK). The BDNF level was determined by means of quantitative real-time PCR and normalized to the level of GAPDH mRNA and β2M. The values are the averages of six independent PCR experiments. The boundary of the box closest to zero indicates the 25th percentile, the line within the box marks the median, and the boundary of the box farthest from zero indicates the 75th percentile.