A BAC clone containing the entire Pvalb gene and approximately 150 kb of upstream and 25 kb downstream regions was used for BAC modifications. A GFP expression cassette was inserted in the first coding exon at the translation initiation site. Five transgenic founders were identified by PCR and confirmed by southern blotting. In the cerebellum, different fractions of Purkinje, basket, and stellate neurons expressed GFP among different transgenic lines, from a few percent (the B20 line) to near 100% (the B13 line, and unpublished data), likely due to different genomic integration sites of the transgene. The onset of GFP usually started in the late second postnatal week and increased to higher levels by the fourth week.