The Arabidopsis (Arabidopsis thaliana) genome contains 15 genes encoding protein homologs of the barley mildew resistance locus o (MLO) protein biochemically shown to have a seven-transmembrane domain topology and localize to the plasma membrane. Towards elucidating the functions of MLOs, the largest family of seven-transmembrane domain proteins specific to plants, we comprehensively determined AtMLO gene expression patterns by a combination of experimental and in silico studies. Experimentation comprised analyses of transgenic Arabidopsis lines bearing promoter::Beta-glucuronidase (GUS) transcriptional fusions as well as semi-quantitative determination of transcripts by reverse transcription coupled to polymerase chain reaction (RT-PCR). These results were combined with information extracted from public gene profiling databases, and compared to the expression patterns of genes encoding the heterotrimeric G-protein subunits. We found that each AtMLO gene has a unique expression pattern and is regulated differently by a variety of biotic and/or abiotic stimuli, suggesting that AtMLO proteins function in diverse developmental and response processes. The expression of several phylogenetically closely-related AtMLO genes showed similar or overlapping tissue specificity and analogous responsiveness to external stimuli, suggesting functional redundancy, co-function, or antagonistic function(s).