The Igh 3' regulatory region (3'RR) controls class switch recombination (CSR) and somatic hypermutation (SHM) in B cells. The mouse 3'RR contains four enhancer elements with hs1,2 (Rr153) flanked by inverted repeated intervening sequences (IRIS) and the center of a 25-kb palindrome bounded by two hs3 enhancer inverted copies (hs3a (Rr154) and hs3b (Rr155)). Enhancer hs4 (Rr156) lies downstream of the palindrome. This allele has a deletion of the left part of the 3'RR (hs3a-IRIS-hs1,2). (J:234741)