Different regions of the same tissue specimen can give highly similar or highly discordant expression profiles. Shown are scatter plots of the expression profiles of two different regions of the same muscle biopsy from patient 6 (Panel A), and patient 4 (Panel B). Only "present" calls are shown (~2,000 of the 4,600 probe sets studied). An example of one patient showing very high concordance between two different biopsy regions is shown (panel A), and an example of a second patient showing very poor correlation between the two biopsy fragments (panel B). The solid lines indicate two-, three-, ten- and thirty-fold difference thresholds.