POSTN and cancer: Apart from the often implicit issue of periostin isoform expression, several aspects of periostin are controversially described and discussed in the literature to date, prominently among them: (i) periostin subcellular localization, (ii) the cell types responsible for periostin expression and the physiological sites of periostin protein localization post secretion, and (iii), in the cancer literature, periostin function being tumor promoting or suppressing.