ES cells from homozygous Fn1tm1Ksek male mouse in which exon EDB (EIIIB) was deleted and replaced with a singe loxp site was isolated and targeted with the same construct used in Fn1tm1Bwg. Following homologous recombination and transient cre-mediated recombination, a single loxp site was left in place of the exon encoding the EIIIA domain. Northern and Western blot confirmed the absence of EIII1 and EIIIB mRNA and protein and that deletion of both of those exons did not compromise the expression of FN mRNA and protein, nor the deposition of FN into the matrix.