Insect and vertebrate Porcupine genes encode multi-pass endoplasmic reticulum proteins involved in the processing of Wnt (wingless and int homologue) proteins, a class of secreted glycoprotein factors homologous to the Drosophila melanogaster segment polarity gene Wingless (Wg). Here we report the cloning of cDNAs encoding the human homologue of the Drosophila gene Porcupine (Porc), the characterization of its genomic structure and the quantitative analysis of its expression in a comprehensive panel of human tissues. The human Porcupine locus (MG61/PORC) spans 15 exons over approximately 12 kb of genomic sequence on Xp11.23. Real-time quantitative expression analysis reveals that MG61/PORC transcripts are expressed in multiple tissues, but are particularly abundant in the brain. Like its mouse and Xenopus homologues, MG61/PORC encodes four protein isoforms (A-D) generated through alternative splicing and expressed in a tissue-specific fashion. Finally, we present evidence indicating that MG61/PORC can influence the activity of a human Wnt7A expression construct in a T-cell factor-responsive reporter assay.