A floxed neo was inserted in intron 5 and a loxP site in intron 7. Transient Cre expression deleted the larger floxed region, including exons 6 and 7. These exons encode amino acids 254-380, which comprise the N-terminal portion of the rel DNA-binding domain (DBD). Germ line transmission was verified by PCR and Southern blot. RNA and protein expression in mutant MEFs showed that the allele encodes a truncated protein containing an internal deletion as expected. Immunoreactivity was eliminated as demonstrated by using antisera against the DBD. (J:91482)