Heart muscle cell specification (cardiac myogenesis) and creating the four-chambered heart
(cardiac morphogenesis) are subject to regulation, in certain model organisms, by bone
morphogenetic proteins and their receptors. Extrapolation to mammals from organisms that
develop outside the mother (flies, fish, frogs, and avians) has been confounded by very early
lethality—at gastrulation—of many null alleles needed to prove cause–effect relations in this
pathway. Here, we describe the use of lineage-or compartment-restricted null alleles as well …