Dppa4 (developmental pluripotency-associated 4) has been identified in several high-profile screens as a gene that is expressed exclusively in pluripotent cells. It encodes a nuclear protein with an SAP-like domain and appears to be associated preferentially with transcriptionally active chromatin. Its exquisite expression pattern and results of RNA interference experiments have led to speculation that Dppa4, as well as its nearby homolog Dppa2, might play essential roles in embryonic stem (ES) cell function and/or germ cell development. To rigorously assess suggested roles, we have generated Dppa4-deficient and Dppa4/Dppa2 doubly deficient ES cells, as well as mice lacking Dppa4. Contrary to predictions, we find that Dppa4 is completely dispensable for ES cell identity and germ cell development. Instead, loss of Dppa4 in mice results in late embryonic/perinatal death and striking skeletal defects with partial penetrance. Thus, surprisingly, Dppa4-deficiency affects tissues that apparently never transcribed the gene, and at least some loss-of-function defects manifest phenotypically at an embryonic stage long after physiologic Dppa4 expression has ceased. Concomitant with targeted gene inactivation, we have introduced into the Dppa4 locus a red fluorescent marker (tandem-dimer red fluorescent protein) that is compatible with green fluorescent proteins and allows noninvasive visualization of pluripotent cells and reprogramming events.