Transcriptomes are key to understanding the relationship between genotype and phenotype. The ability to infer the expression state (active or inactive) of genes in the transcriptome offers unique benefits for addressing this issue. For example, qualitative changes in gene expression may underly the origin of novel phenotypes, and expression states are readily comparable between tissues and species. However, inferring the expression state of genes is a surprisingly difficult problem, owing to the complex biological and technical processes that give rise to observed transcriptomic datasets. Here, we develop a hierarchical Bayesian mixture model that describes this complex process, and allows us to infer expression state of genes from replicate transcriptomic libraries. We explore the statistical behavior of this method with analyses of simulated datasets-where we demonstrate its ability to correctly infer true (known) expression states-and empirical-benchmark datasets, where we demonstrate that the expression states inferred from RNA-seq datasets using our method are consistent with those based on independent evidence. The power of our method to correctly infer expression states is generally high and, remarkably, approaches the maximum possible power for this inference problem. We present an empirical analysis of primate-brain transcriptomes, which identifies genes that have a unique expression state in humans. Our method is implemented in the freely-available R package zigzag.detecting zero transcripts of a given gene in a given tissue does 53 not necessarily indicate that it is inactive. Third, even when 54 we detect transcripts of a given gene, its measured expression 55 level is likely to vary among libraries owing to both biological 56 factors (e.g., population-level variation) and technical factors 57 (i.e., the relative abundance of a given transcript in a given 58 library depends on the total transcript number of that library).
59Therefore, the rank order in expression level of two genes in 60 one library may differ from their rank order in a second library, 61 which complicates methods that infer the expression state of 62 genes based on fixed expression-level thresholds (17, 21).
63Here, we present a hierarchical Bayesian model that de-64 scribes the biological and technical processes that generate 65 transcriptomic data that-by explicitly accommodating the 66 factors described above-allows us to infer the expression state 67 of each gene from replicate RNA-seq libraries. We present anal-68 yses of simulated datasets that validate the implementation 69 and characterize the statistical behavior of our hierarchical 70 Bayesian model. We also apply our method to several em-71 pirical datasets, and demonstrate that the expression states 72 inferred using our method are consistent with expectations 73 based on independent information, such as epigenetic marks 74 and developmental-genetic studies. Finally, we demonstrate 75 our method with an empirical analysis of primate-brain tran-76 scriptomes that identi...