Smad1 Conditional Knockout mouse models

Bone morphogenetic protein signaling is transduced by Smad1, the mouse ortholog of SMAD1, a receptor-regulated SMAD whose MH1 domain binds GC-rich BMP response elements and whose MH2 domain is phosphorylated at a carboxy-terminal SSVS motif by the type I receptors ALK1, ALK2, ALK3 and ALK6, after which it trimerizes with SMAD4 and enters the nucleus. Signal duration is set in the linker region, where CDK8 and CDK9 prime and GSK3 extends phosphorylation to recruit the ubiquitin ligase SMURF1, terminating output. Germline deletion is embryonic lethal from allantois and chorioallantoic fusion failure, so conditional nulls with endothelial, chondrocyte or germ cell drivers are required for postnatal questions, and linker phospho-dead knockins uniquely test signal duration. Smad5 and Smad9 are genuinely redundant BMP effectors, so compound conditional alleles are usually necessary before concluding that a BMP-dependent process is Smad1 dependent.

Smad1 conditional knockout mice carry a floxed allele so you delete function only where Cre is active. The germline allele stays intact until you cross to a tissue specific Cre. Labs reach for this design when a global knockout is lethal, when they need adult onset loss, or when regional redundancy hides a whole body phenotype.