Nonchemically modified deoxyribonucleotides were used to introduce the (5-ATGCAGAAAATCTTAGAGTAGGCGATCTGGTAAGTCAACA-3) mutation into a mouse ES cell line. This alteration is a substitution of 3 nucleotides (TCC to AAG) at position 185, resulting in an early STOP codon at aa 24 of the encoded protein. (J:237116)