UASt regulatory sequences drive expression of 'TIPF', a sensor that can be used to assay tkv activation. It consists of tkv isoform-D coding sequences fused to the conformation-sensitive circularly permutated yellow fluorescent protein (cpYFP) and FKBP12 (a motif that binds to the unphosphorylated glycine-serine rich domain of inactive type I BMP receptors). Activation of tkv displaces the FKBP12 motif, allowing cpYFP to adopt a fluorescent conformation.