Male mouse urine contains a pregnancy-blocking chemosignal that causes pre-implantation pregnancy failure in recently mated female mice. However, females are able to recognize the chemosignal of the male with which they mated, preventing it from aborting his own offspring. The individuality of the pregnancy-blocking chemosignal is influenced by genes of the major histocompatibility complex (MHC), although the chemical nature of the signal remains unclear. Possible candidates include fragments of MHC proteins, the highly polymorphic major urinary proteins (MUPs) and the profile of low-molecular-mass volatiles, which possess male pheromonal activity in other contexts. A recent study has found a high-molecular-mass fraction of male urine containing MUPs to be ineffective in eliciting pregnancy block. Moreover, both the pregnancy-blocking activity and the individuality of the signal were associated with the low-molecular-mass fraction of male urine.

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