Genetic background affects the severity of the FN.null phenotype. The percentage of more severe (black bars) and less severe (gray bars) FN.null homozygous embryos is presented in relation to the increasing extent of strain C57/BL6 genetic background. As the extent of C57/BL6 genetic background increases, the percent of mutant embryos displaying the more severe phenotype decreases. On pure 129/Sv background (0% C57/BL6), all mutant embryos are of the more severe class, lacking overt heart development. In contrast, by the sixth breeding generation to the C57/BL6 strain (98% C57/BL6), all mutant embryos display the less severe phenotype, with a visibly detectable primitive heart.