While investigating microRNA targets, we have found that human genes divide into two roughly equal populations, based on the fraction of A plus T bases in their 3' UTRs. Using the Gene Ontology database, we find significant functional differences between the two gene populations, with AT-rich genes implicated in transcription and translation processes, and GC-rich genes implicated in signal transduction and posttranslational protein modification. Better understanding of the background distribution of nucleotides in 3' UTRs may allow improved prediction of microRNA-targeted genes in humans. We predict at least 1,200 KnownGene transcripts to be regulated by microRNAs. The large majority of these microRNA targets are in the AT-rich 3' UTR population. However, notwithstanding this preference for AT-rich targets, microRNA targets are found preferentially to be regulatory genes themselves, including both transcription factors and posttranslational modifiers. These results suggest that some processes involving mRNA, of which microRNA regulation may be just one, require AT-richness of 3' UTRs for functionality. A relationship, not simply one-to-one, between these 3' UTR populations and large-scale genomic isochores is described.