The sorting and delivery of membrane and secretory proteins to the apical domain of epithelial cells remain rich fields for investigation. The different classes of apical membrane proteins have distinct targeting signals within their structure, but most signals have not yet been identified. The single, transmembrane proteins have been studied in some detail and their routing to the apical surface differs among epithelial cells. This difference can be exploited in the search for signals. Additionally, the different secretory responses of cells to microtubule disruption may reveal further insights into the dynamics of the apical domain.