Serine codon 23 (TCA) in exon 2 was changed to alanine (GCA) (p.S23A). Transient cre recombinase expression excised a loxP site flanked puromycin resistance gene cassette that was inserted into intron1. RT-PCR of mutant MEFs confirmed correct targeting. The mutation prevents the phosphorylation of the residue in the encoded peptide.