TOKYO – "This May 13, buy your mom a Nintendo DS."

With Mother's Day fast approaching, Tokyo's subways and billboards are plastered with advertisements for chocolate and flowers. But one of the most visible ads, running every few minutes on the TV monitors inside subway cars, suggests that you give your mom Nintendo's portable videogame system and load it with software like Nintendogs, Brain Age or an interactive cookbook called 1000 Recipes.

It's likely that more than a few consumers will take Nintendo up on the offer. Since it launched in 2004, the touch-screen Nintendo DS has sold well over 16 million units in Japan, outpacing every other game console on the market. On the strength of games like puppy-raising simulators and brain-training ware, many of these sales have been to women. Initially taken off guard, Japan's game makers are now in the grip of girl fever – scrambling to market directly to women gamers who were once ignored.

"I'd go to school and play the old gray Game Boy on the train, and people would stare," says Eleanor Harada, a 31-year-old computer programmer at a financial firm. "It wasn't normal. Whereas now, if you're a female and you're on the train, you can take out a DS. I think it helps that it looks more stylish than it used to."

Style is a big piece of the marketing surge. At Tsutaya, one of Japan's largest media retail chains, signs reading "Girls' Style" hang in the DS section, explaining to potential buyers what games are most popular with women. In great part, they are the same titles popular with men: New Super Mario Bros., Final Fantasy III, English Training. Nintendo has been reticent about the exact numbers, but in late 2005 Nintendo president Satoru Iwata said about a third of Brain Age buyers in Japan were women.

But in growing numbers, titles are being crafted specifically with women in mind. Following the surprise cross-gender success of Nintendogs, several of these are pet simulators. Magazines like Sutekina Okusan (Fantastic Wife), aimed at middle-aged housewives, advertise Nintendo DS games like Hamusutā to Kurasou, which translates to, "Let's Spend Time With Hamsters."

Self-improvement games comprise another genre aimed at the female audience – titles like My Happy Manner Book, an interactive Miss Manners-style guide to social situations. Nintendo has had great success with a pair of interactive cookbooks: Cooking Navi has sold more than 800,000 copies, and 1000 Recipes more than 300,000. A game from Sega for young girls called Love and Berry has sold 1 million-plus copies.

Even when game designers aren't making games specifically for women, they've learned the importance of keeping that rapidly emerging market in mind, says Keiichi Yano, designer of the Nintendo DS game Elite Beat Agents.

"When we come up with new project ideas, we take into consideration what the female gamer community is interested in and what DS games are selling for female users," Yano says. In Japan, anything that in sells women's magazines – "learning, manners, food" – can also move games off the shelves, says Keiichi.

One indication that women are buying DS in droves is the cottage industry that has sprung up almost overnight for stylish protective DS pouches. Of course, DS owners both male and female need slipcases to protect the system, which costs about $160 in Japan. But the abundance of styles and colors are clearly aimed at women. Some are like miniature purses, bedecked with colorful plastic jewels. There are drawstring pouches in gold lamé, and elegant covers made of traditional kimono cloth. These can run upwards of $30 or $40 each.

Sony, Nintendo's biggest competitor in Japan, is by and large missing out on the women-gamers boom. Harada says the only people she's ever seen on the train playing PSP have been men. The PlayStation products still have the image here of being for hardcore gamers, while the DS – with its simple input methods and unique software – is seen as more accessible to beginning players.

"My friend who sits next to me (at work) was saying she might buy a DS for Common Sense Training," Harada says. "When I asked her why, she said it's because it doesn't seem difficult."

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