Genomic imprinting - parental-specific gene expression in which either the maternal or paternal copy of a gene is expressed - is important in early development and can contribute to disease. To date, imprinting has been mostly studied in mammalian embryos, which often entails long experimental time frames. Now, on p. 437,Denise Barlow and colleagues demonstrate that differentiating embryonic stem(ES) cells provide a much-needed in vitro system for studying this phenomenon. The authors show that imprinted expression of the Igf2r gene is established when ES cells differentiate, and that this coincides with the onset of the paternal-specific expression of Airn, a non-coding RNA that, in the mouse, silences the paternal allele of Igf2r. These events mimic aspects of imprinting that occur in the mouse embryo during and after implantation. Surprisingly, during ES cell differentiation, expression from the paternal Igf2r promoter remains constant instead of being silenced, while expression from the maternal Igf2r promoter increases up to tenfold, revealing a novel mechanism for Airn-mediated imprinting - through an expression bias rather than silencing.