Necrotic cell death has long been regarded as an uncontrolled consequence of chemical or mechanical insult. Recent work has revealed it to be intricately programmed and deliberate in certain scenarios like infection. This programmed necrosis or ‘necroptosis’ is triggered by a cascade of intracellular signals culminating in the phosphorylation of the pseudokinase Mixed Lineage Kinase domain-Like (MLKL). We have identified a mouse strain with a single base pair missense, gain of function mutation in the gene encoding MLKL. Mice homozygous for this mutant allele, MlklPlt15, are born at expected Mendelian ratios and are indistinguishable from wild type littermates, but fail to thrive and die 2-6 days after birth. Despite normal levels of Mlkl mRNA, MLKL protein is undetectable by Western blot in MlklPlt15/Plt15 embryos or pups. This presents an interesting conundrum; full Mlklgene knock out mice are born at expected Mendelian ratios and thrive into adulthood. So why do MlklPlt15/Plt15 pups that are ostensibly MLKL deficient at the protein level, die shortly after birth? We show that they die because there are natural mechanisms that can suppress levels of activated MLKL in a cell, and that these mechanisms are overcome when exposure to certain cytokines trigger increase Mlkl gene expression shortly before and after birth. Being born in ‘germ free’ housing can extend the life of MlklPlt15/Plt15 pups by several days, indicating that colonization with normal microflaura is one of these natural triggers of Mlkl gene expression. The MlklPlt15 mouse strain therefore provides an exciting animal model for investigating necroptosis at the molecular, cellular and physiological level.