pICln is a 26-kDa protein that is ubiquitously expressed and highly conserved from Xenopus laevis to Homo sapiens. The physiological functions of pICln remain to be established. To address this question, we disrupted the ICln gene in embryonic stem cells. We found that murine embryos lacking ICln die early in gestation (between stages E3.5 and E7.5). Furthermore, we found that ICln is essential for embryonic stem cell viability. Previously, we showed that pICln interacts directly with a homolog of a yeast protein that binds a PAK-like kinase and participates in the regulation of cell morphology and cell cycling, pICln also forms a complex with: several core spliceosomal proteins, and this interaction may play a role in the regulation of spliceosomal biogenesis. Collectively, these data strongly suggest that pICln participates in critical cellular pathways, including regulation of the cell cycle and RNA processing.