G rowing evidence suggests that genes are not expressed at steady state levels, but instead, gene transcription has a stochastic component. We review recent literature concerning stochastic gene expression and discuss our model in which allele copy number influenced the probability that the product level o f a gene will fall below a critical level in an individual cell over time. In this model, loss o f a single allele caused auto-regulated genes to be permanendy inactivated. Implications for haploinsufficent phenotypes and redundant gene families are discussed. Specific genes that have been associated with haploinsufficiency are considered in the context o f emerging studies and models o f stochastic gene expression.