Mice, KnockoutPhenotypeGene Knockout TechniquesMutationMice, Inbred C57BLMolecular Sequence DataGene DeletionMice, TransgenicDisease Models, AnimalGenotypeBase SequenceCells, CulturedSignal TransductionAmino Acid SequenceCell DifferentiationRNA, MessengerAllelesGene Expression RegulationGene Targeting