Why is msg called a carcinogen?

Don't know why: Monosodium glutamate (msg) can be neurotoxic because of it's resemblence to a natural exitatory neurotransmitter called glutatmate. Glutamate when too abundant can cause calcium to influx into the cell too quickly and induced apoptosis or programmed cell death. Msg can do the same. Don't know if it has been classified as a carcinogen however. Please enlighten me if you find any info on this.