Nature: The enzyme protein kinase M-ζ (PKM-ζ) was believed to be key to long-term memory after a series of studies showed that blocking the enzyme in the brains of mice caused them to forget old memories. Now, two independent studies have shown that the connection between PKM-ζ and memory may not be as significant as previously believed. A group led by Richard Huganir of Johns Hopkins University in Baltimore, Maryland, and another led by Robert Messing from the University of California, San Francisco, independently created mice strains in which PKM-ζ was not produced. Both groups found that the mice showed no signs of memory impairment, either in the forming of memories or in recalling them. Todd Sacktor of SUNY Downstate Medical Center in New York City, who originally discovered the apparent connection between PKM-ζ and long-term memory, believes that related proteins may have served the same function as PKM-ζ in the altered mice. However, Huganir’s group, which also created mice whose PKM-ζ production could be turned off in adulthood, found that there was no loss of memory ability in these mice either. Although PKM-ζ has not been completely ruled out as having a role in memory development and retention, the new studies have shown that the system is much more complicated than it originally appeared.