Twenty-five years ago forensic investigators needed a blood or semen stain the size of a dime to extract the DNA of a suspect. In the 1990s they needed a speck. Today they need what can’t be seen—as few as 70 of the 400,000 skin cells we shed each day. “Touch” or “trace” DNA has typically been used to investigate violent crimes. In 2008 a lab in Virginia extracted the DNA of a still-unidentified male from the long johns JonBenet Ramsey wore on the night of her death. Last year police in Wasatch County, Utah arrested a suspect for a 1995 murder after obtaining DNA from a cigarette he had discarded while officers were tailing him. The sample matched trace DNA recovered from granite rocks the killer used to bludgeon his 17-year-old victim.

Buoyed by that type of success, investigators are now preserving DNA from crime scenes where perps haven’t bled or ejaculated. In St. Petersburg, Florida police say trace DNA has led to arrests in 38 percent of burglaries over the past three years. In Houston about 75 percent of the more than 3,000 matches since 2008 have involved property crimes. In New York City about a third of DNA cases are related to property crimes.

Because discarded skin cells are dead, scientists believe that sweat, which picks up free-floating strands as it moves through pores, is the key to trace-DNA readings. A scientist needs about a nanogram of DNA to work up a profile. Holding a glass for 60 seconds yields about half that, while touching fabric or wood for a minute is more than sufficient, as is rubbing cotton against a palm or finger for 15 seconds. The more pressure and friction applied, the more likely DNA will be left. The rougher the surface, the more cells will stay. Besides rocks, investigators have obtained trace samples from pistol grips, pocket linings, asphalt, shoestrings and a victim’s tongue.

Skin cells are easily transferred, so one can’t assume that finding a person’s DNA on an object means he or she touched it. One investigator swabbed his own hand after shaking hands at a party, and a lab was able to extract the DNA of two of the people he’d met. There is a risk that juries will fail to heed that disclaimer and innocent people will be convicted based on cells that migrated to a stolen item or a murder weapon. The fact that a person can be tagged so easily perhaps makes the report that Madonna takes a cleanup crew on tour with her sound less bizarre. “We can only enter [her dressing room] after her sterilization team has left,” a promoter in Portugal told the Daily Mirror. “There will not be any of Madonna’s DNA, any hair or anything.”


British software developer Robb Lewis earlier this year launched after seeing tweets about the difficulty of closing a Skype account. He ranks dozens of sites on how hard they are to leave without a trace. Among your friends for life: Blogger, Gawker, GoDaddy, Netflix, Pinterest and Starbucks. The tough but possible include Amazon, Craigslist, iTunes, Pandora and Ticketmaster. To return to your pre-1994 existence, here‘s how to unplug from four popular services.

FACEBOOK Click on the account menu at top right. Select “Account Settings.” Click “Security.” Click “Deactivate your account.” Facebook will save your timeline (i.e., friends, photos, posts) indefinitely “in case you want to come back.” To vaporize that info, visit Messages you have sent will always be visible.

GOOGLE Under “Account Management,” click on “Delete account and data.” You will be shown a list of products (e.g., Gmail, Blogger, YouTube). Click the box next to each to acknowledge you are selecting the nuclear option. Enter your password and check the button for “Yes, I want to delete my account” and the one confirming you don’t owe Google money. Click the “Delete Google account” button. Google will preserve your footprint for a “limited window of time” before destruction.

TWITTER Sign in at Go to “Settings” under the gear icon. Click on “Deactivate my account.” Click the “Deactivate @[user name]” button. Enter your password. Twitter stores your tweets for 30 days before wiping them.

BLOGGER You can’t delete your account without also deleting your Google account, but you can create the same effect by removing your blogs and editing your profile to delete personal data (though residual copies may remain on backup systems). For required fields, you should enter false information, essentially a version of witness protection. To permanently disengage, uncheck the “Share my profile” button under “Edit Profile.” That will shut you out, though your bits will remain in digital orbit, presumably forever.