Laminins are a family of basement membrane proteins with diverse roles in fundamental developmental processes such as epiblast polarization and gastrulation, as well as in organ development and function. We have focused on the laminin alpha1 and alpha5 chains, the ancestral laminin alpha chains required for development. To elucidate the unique functions of laminin alpha1 and alpha5 and their COOH-terminal LG domains, we have produced a collection of laminin knockout and transgenic mice expressing full length and chimeric laminin alpha5/alpha1 chains. Crossing the transgenes onto the Lama5-/- background generates "pseudo-knockins", so called because endogenous laminin alpha5 is replaced by transgene-encoded proteins. Expression of a chimera with the entire alpha5LG domain replaced by alpha1LG had minimal ameliorative effects on the defects observed in Lama5-/- embryos. In contrast, high level expression of a chimera with only the alpha5LG3-5 tandem replaced by alpha1LG3-5 completely rescued defects in digit septation, neural tube closure, placental labyrinth morphology, lung lobe septation, hair growth, and vascularization of kidney glomeruli. These mice were viable for several months, but they developed a lethal nephrotic syndrome. Our results show that: (1) the laminin alpha5LG1-2 tandem plays an essential role during development and harbors the great majority of the functionality of the alpha5LG domain; and (2) the alpha5LG3-5 tandem serves as a novel determinant required for the kidney's glomerular filtration barrier to plasma protein.