We read with great interest the excellent review by Maherali and Hochedlinger (2008) that recommends standards for characterization of pluripotent stem cell lines, especially the many new lines being generated using factor-based reprogramming techniques (induced pluripotent stem cells, or iPSCs). Of note was the suggestion that iPSCs should be assessed for ‘‘functional differentiation through the highest-stringency test acceptable.’’For murine iPSCs, this means germline transmission following blastocyst chimerism, and for human iPSCs this means assessment of teratoma pathology. Given the fast pace of discovery in the field, the value and relevance of time-consuming characterization of cell lines are bound to be debated. We’d like to highlight what’s at risk when the pressure for rapid publication erodes the imperative for applying rigorous and uniform standards before assigning the label ‘‘iPSC’’to novel cell lines.

The term ‘‘pluripotency’’can be assigned according to lax or stringent criteria. A diverse array of stem cell types have been labeled pluripotent: multipotential adult progenitor cells (MAPCs); amniotic fluid-derived stem cells (AFS); marrow-isolated adult multilineage inducible cells (MIAMI); testes-derived stem cells; and a variety of embryonic stem cells derived by parthenogenesis, blastomere culture, and somatic cell nuclear transfer. In the loosest sense, a pluripotent cell includes in its progeny elements of all three embryonic germ layers (ectoderm, endoderm, and mesoderm), regardless of experimental context. In the strictest sense, pluripotency pertains to cells