Nespas is the non-coding antisense transcript of the imprinted Gnas cluster; it is expressed from the paternal allele and is located on mouse distal chromosome 2. In this thesis new transcripts of >10 kb and 0.8 kb have been identified. The 0.8 kb transcript is a spliced variant that is retained in the nucleus and its 3’ end lies approximately 30 kb from the start site. Transcription from the Nespas promoter does not proceed beyond this point. A collection of previously known splice variants have also been detected and are exported to the cytoplasm. Nespas is expressed in the embryo during the second half of gestation and peaks at 13.5 dpc. Nespas is imprinted in the placenta at 11.5, 15.5 and 17.5 dpc. The Nespastm4Jop allele, to truncate the Nespas transcript 10.5 kb from the start site, has been transmitted through the germline and a breeding colony established. Preliminary analysis shows Nespas has a regulatory function. A second targeting construct to truncate Nespas 12.5 kb from the start site has been designed and assembled to investigate whether the 3’ end of the Nespas transcript that is transcribed upstream of the Nesp promoter is required for Nespas-mediated silencing of Nesp.