As part of a systematic screen for novel imprinted genes of human chromosome 7 we have investigated GRB10, which belongs to a small family of adapter proteins, known to interact with a number of receptor tyrosine kinases and signalling molecules. Upon allele-specific transcription analysis involving multiple distinct splice variants in various fetal tissues, we found that human GRB10 is imprinted in a highly isoform- and tissue-specific manner. In fetal brains, most variants are transcribed exclusively from the paternal allele. Imprinted expression in this tissue is not accompanied by allele-specific methylation of the most 5' CpG island. In skeletal muscle, one GRB10 isoform, gamma1, is expressed from the maternal allele alone, whereas in numerous other fetal tissues, all GRB10 splice variants are transcribed from both parental alleles. A remarkable finding is paternal-specific expression of GRB10 in the human fetal brain, since, in the mouse, this gene is transcribed exclusively from the maternal allele. To our knowledge, this is the first example of a gene that is oppositely imprinted in mouse and human.