The description contained important information about a microarray sample. Third, the expression profiles had to be obtained using normal tissue samples. Microarray profiles of cancer cells or dis eased tissues were excluded from selection. Fourth, the tissue sample used for microarray profiling should not be cultured Dasatinib Src in vitro or treated with any drugs before RNA extraction. No expression profiles of primary or secondary cell cultures were selected for this study. By following the above criteria, we compiled 3,030 microarray gene expression profiles across a variety of human tissues. The number of selected profiles varied among tissues, depending on data availability. An attempt was made to include as many tissues as possible, even though some tissues had only a few expression pro files available in the GEO database.

Nevertheless, some tis sues had a relatively large number of expression profiles, and were thus particularly suited for identifying tissue selective genes. For instance, there were 645 brain gene expression profiles. These expression profiles were obtained from various regions of postmortem brain such as entorhinal cortex, hippocampus and cerebellum, and could be used to iden tify genes specifically expressed in neurons. Microarray data normalization and integration Microarray raw data in CEL file format were down loaded from the GEO database, and then normalized by One challenging task in this study was to combine the expression profiles of various tissue types and from dif ferent microarray studies into a single integrated dataset.

As outlined in Figure 1, our approach included the fol lowing steps. First, the selected microarray CEL files were organized into different normalization groups, each of which contained expression profiles of the same or similar tissue type. For example, one normalization group was consisted of 117 liver microarray profiles, whereas another group contained 112 expression pro files of six endocrine glands, including pituitary gland, thyroid gland, parathyroid gland, thymus gland, adrenal gland and pancreas. Within a normalization group, the variation of tissue type was thus minimized although the expression profiles were nevertheless obtained from different microarray studies. Second, each group of microarray profiles was normal ized by using the invariant set method.

For each nor malization Dacomitinib group, the expression profile with median overall intensity was chosen as the baseline array, against which the other profiles were normalized at probe inten sity level. A subset of PM probes with small rank differ ence between the profile to be normalized and the baseline array were chosen as the invariant set for fitting a normalization curve. The normalization transformation was then performed for all the probes in the profile based on the curve.