You Try to Generate a Knockout Mouse for a Gene

You try to generate a knockout mouse for a gene of interest, using the protocol described in this chapter. You do obtain progeny heterozygous for the knockout, as assessed by Southern blot. When you cross these heterozygous mice, you obtain viable progeny - some are heterozygous for the knockout, but none are found that are homozygous for the knockout. Which of following is the MOST likely explanation for this result?

A) The target gene was not disrupted successfully.
B) Genes in addition to the target gene were disrupted.
C) The heterozygous mice in your cross did not mate properly.
D) The target gene is haploinsufficient; that is, a mutant phenotype is observed when only one copy of the gene is disrupted.
E) The target gene is essential for viability.