The callipyge mutation in sheep results in postnatal hypertrophy and leanness of skeletal muscles in the pelvic limbs and loins. Associated changes also occur in the expression of a number of imprinted genes flanking the site of the mutation, which lies at the telomeric end of ovine chromosome 18. The transcripts from several of these genes are either spliced or undergo substantial RNA processing, sometimes in a very complex manner. The current investigation examined the effects of the callipyge mutation on the relative expression of some of these splice variants in samples taken: at birth, when the muscle hypertrophy phenotype is not expressed; and at 12 weeks of age, when the phenotype is fully apparent. It was concluded that changes in the postnatal developmental expression pattern of Dlk-1 are closely associated with the expression of the phenotype and that the callipyge mutation may promote a fetal-like gene expression program for some genes during postnatal life.