Generation of a preliminary bovine gene atlas, using expression clustering to annotate gene function

Genes whose products function in a common biologi- cal process are often co-regulated. When regulation occurs at the transcriptional level, co-expressed genes can be detected globally by expression arrays or by sequencing non-normalized cDNA li- braries. We examined bovine gene expression in 27 tissues using non-normalized cDNA library sequencing. Contigs were generated from expressed sequence tags whose sequences overlapped. Contigs containing a minimum of five expressed sequence tags were ordered via a hierarchical clustering process, where the distance between the contigs represents their expression pattern similarity across tis- sues. Gene ontology terms associated with the genes in each cluster showed that co-clustered genes encoded proteins involved in a com- mon biological process. This process can be used to annotate genes of unknown function in the cluster. Gene expression was compared between bovine and human tissues; there were significant correla- tions between species for each tissue, with the exception of thy- roid and placenta. Tissues were also clustered based on the genes they express; tissues with similar physiological functions clustered