The endogenous N-terminal Ribeye A-domain was replaced with a loxP site followed by the Ribeye A domain cDNA fused to GCaMP3 (a GFP based calcium sensor). The coding region is followed by a polyA signal, an FRT-flanked PGK-neomycin selection cassette, and a 3' prime loxP site. This knock-in allele expresses a tagged C-terminally truncated Ribeye isoform without the B-domain. (J:233453)