Although Les Claypool's twisted sense of humor has fueled Primus' best-known work, the singer and bassist is deeply serious about his two-year-old nephew Matthew, who is battling leukemia. Starting today, Claypool is auctioning off a prototype bass that he has been playing in concert for the past year, and using the proceeds to help pay for Matthew's mounting medical costs.

"The expenses of these things are insane," Claypool tells Rolling Stone. "I remember he had to get a couple of shots when he first got diagnosed with this, and the shots were $80,000 apiece. And that's just the tip of the iceberg. It's been nonstop."

Matthew is responding well to a bone marrow transplant he underwent two months ago, Claypool said, though he's been affected with graft-versus-host disease, a common, though dangerous, complication from transplants. "They can't really treat it until he's 100 percent cancer-free," Claypool says.

Primus has been playing benefit shows and raising money for Matthew, and Claypool figured he could attract additional attention by auctioning a "very high-end, very expensive" prototype bass he had designed to his specifications. The four-string Les Claypool Pachyderm has a walnut top and a maple back, with LED lights in the side of the neck. "It's the most comfortable playing four-string bass that I've ever played in my entire life," Claypool says.

Primus recently announced a fall tour featuring a 3D-enhanced stage show with a four-way surround-sound system. "We want to take the notion of modern acid rock to the next level," Claypool says. "The heads are demanding more, so we're going 3D on them. There's going to be various textures and imagery leaping into your face as we noodle our little fingers away for the masses."

The tour starts October 12th, which Claypool hopes will coincide with his nephew's release from the hospital. "Once Matthew is 100 days in [from the bone marrow transplant], they feel like he's over the hump and he can potentially go home," Claypool says. "Which would be an amazing thing, because he hasn't been home in a long time. He's a champ – it's unbelievable how tough this little kid is. It's almost like all he's known his whole life has been pain."

For more information or to donate money, visit www.babymatthew.org.

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