The genes for β-actin (top left) and γ-actin protein (top right) are similar in many ways, as are the mRNAs and proteins they code for, but the small differences between them are important. At the level of genes, both intron 3 (shown in red for β-actin and orange for γ-actin) and the 3´ untranslated region (3´ UTR) are different: moreover, these two elements are conserved features that are found in both birds and mammals. β-actin also has four codons (not shown) that code for four amino acids that are not found in the γ-actin gene or protein. (Exons, the regions of DNA that are transcribed to produce mRNA, are shown in pale blue). At the level of mRNA, β-actin has a high density of ribosomes (shown in green), whereas γ-actin has a low density; β-actin also has a sequence (grey) that directs the mRNA molecules to the periphery of the cell. At the level of proteins, it has been predicted that β-actin is produced in rapid bursts, whereas γ-actin is produced steadily at a low rate. The essential nature of the differences between β-actin and γ-actin at these three levels is the subject of intensive ongoing research. We thank Yao Wang for preparing this figure.