We report here the identification and partial characterization of a human gene (designated AHNAK) that encodes an unusually large protein (approximately 700 kDa). AHNAK is expressed by means of a 17.5-kilobase mRNA in diverse cellular lineages but is typically repressed in cell lines derived from human neuroblastomas and in several other types of tumors. Unique-sequence domains at the two ends of the protein flank a large internal domain (approximately 4300 amino acids) composed of highly conserved repeated elements, most of which are 128 amino acids in length. The repeated elements in turn display a redundant motif, marked by the recurrence of proline at every seventh residue. Within these sequences, hydrophobic and hydrophilic residues alternate in a manner that is incompatible with a helical coiled-coil structure. Instead, we propose a structure resembling a beta-strand but with a periodicity of 2.33. The structure would engender a polyionic rod approximately 1.2 microns long. Preliminary evidence indicates that the protein resides predominantly within the nucleus, but no function has yet been discerned.