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levels in gene probes. To our knowledge this is the first study that provides such
a framework for analyzing expression data.
Although theoretically gamma distributions are capable of modeling skewed
distributions, our experiments showed that lognormal appears to be more suit-
able in modeling the marginal distribution of gene expression. We also showed
that amongst the two model selection criteria we used, BIC is more accurate in
selecting the number of components for lognormal and gamma mixtures. AIC
on the other hand tends to over estimate the number of components.
We hypothesize that different functional categories of genes ( e .g. transcrip-
tion factors, kinases, structural proteins, etc) may show similar marginal distri-
butions. Unfortunately this expectation is not clearly supported by our study.
Only the single, vague gene ontology term intracellular was found to be over-
represented in both datasets. We believe follow-up experiments are necessary to
determine if this is a due to the quantity/quality of the expression data used, a
deficiency in our methodology, or whether our hypothesis is simply wrong.
To achieve more definitive results we are now preparing to analyze a much
larger dataset including multiple GEO datasets. This will be essential to sample
the expression probes at the resolution needed to accurately model multimodal
marginal distributions. Our results should provide some guidance in the develop-
ment of informed priors or gene specific normalization for use with gene network
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