Mice homozygous for the targeted mutation are viable, develop normally, and are fertile, but do not lactate. Mammary ductal development through pregnancy is normal, but lobuloalveolar development is severely reduced and there is no milk secretion even after prolonged suckling. Levels of the closely related STAT5B signalling molecule are also markedly reduced in STAT5A-deficient mice, but levels increase and phosphorylation is evident after 3 days of suckling. Expression of several milk protein genes is unaffected in homozygous mutants, whereas whey acidic protein expression is severely reduced. Homozygous males exhibit abnormal prostate morphology and an increase in probasin secretion. Immmunological defects exhibited by homozygotes include a reduced number of peritoneal mast cells, decreased IL-3 dependent development of bone marrow-derived mast cells, defective bone marrow-derived macrophage GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor) induced proliferation, decreased number of NK cells and decreased expansion of T cells. This mutant mouse strain may be useful in studies of mammary gland development, lactogenesis, hematopoiesis, and immunological intracellular signal transduction.

 In an attempt to offer alleles on well-characterized or multiple genetic backgrounds, alleles are frequently moved to a genetic background different from that on which an allele was first characterized. This is the case for the strain above. It should be noted that the phenotype could vary from that originally described. We will modify the strain description if necessary as published results become available.

These Stat5a knock-out mice exhibit reduction in lobuloalveolar development with no milk secretion even after prolonged suckling.

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