Two rel-containing cDNA clones were isolated from a library derived from the Daudi human cell line, which is known to express c-rel mRNA. Clone #1 appeared to contain the entire c-rel coding sequence, which differs from v-rel in having three additional N-terminal residues and 111 additional C-terminal residues. In addition, Clone #1 had an internal 32 amino acid exon not found in v-rel or in turkey c-rel. Clone #2 was truncated at its 5' end and did not contain this new exon. Analysis of a genomic clone of human c-rel revealed that the new exon was a portion of an inverted Alu repeat. The occurrence of potential splice sites and of open reading frames in the inverted consensus Alu sequence suggests that the incorporation of Alu fragments as potential coding exons could be a relatively common event in human mRNAs. Whether such messages can be translated is unknown: antiserum raised against a peptide at the predicted C-terminus of the c-rel protein precipitated p82hc-rel, but antiserum raised against a peptide located in the Alu exon did not.