A bacterial artificial chromosome (BAC) library from CHORI was used to obtain a BAC (#RP24-400B5) containing the entire mouse Csk (c-src tyrosine kinase) gene. A point mutation was introduced in Csk at position 266 resulting in a threonine to glycine mutation, inducing a larger ATP-binding pocket to be formed in the expressed protein. To inactivate other genes present in the BAC, both ends of the BAC were deleted, leaving 74kb of sequence, and an in-frame stop codon was inserted in exon 2 of Lman1l (lectin, mannose-binding 1 like) gene.