June 25 marks the 25th anniversary of the death of original Red Hot Chili Peppers guitarist Hillel Slovak, who lost his battle with heroin addiction just as the band started to show its commercial potential.

The Israeli-born Slovak played on two Chili Peppers albums — 1985's Freaky Styley and 1987's The Uplift Mofo Party Plan — before succumbing to an overdose of heroin in 1988 at the age of 26. He was replaced by the then 18-year-old John Frusciante, a disciple of Slovak's playing. It was with Frusciante that the Peppers recorded their breakthrough album, 1989's Mother's Milk, which became their first gold album.

Here's a look back a five clips that show the impact of Slovak.

Before Slovak and drummer Jack Irons gained notoriety with the Peppers, they were both members of What Is This?, a now forgotten L.A.-based quartet fronted by Alain Johannes; who would go on to some success with his own band, Eleven, and performing and recording with Queens of the Stone Age.

Like the Chili Peppers, What Is This? Was formed straight outta Fairfax High School and could actually be considered the Chili Peppers' sister band, since Red Hot bassist Flea was once a member of its predecessor, Anthym. Here's the interesting thing — Slovak and Irons were members of What Is This? and the Peppers simultaneously, but opted to quit the Peppers since What Is This? secured a contract with MCA prior to the Peppers' signing with EMI.

Not to get all Pat O'Brien on you, but I was lucky enough to interview the members of What Is This? in 1984 for the long-defunct BAM magazine at Ernie's Taco House in North Hollywood, upon the release of the band's debut EP, Squeezed. Slovak's brilliant playing is all over the five-song EP, yet his association with What Is This? didn't last long. Though he's featured on all but three songs on the band's 1985 self-titled album (which was produced by Todd Rundgren), he quit the band midway through the sessions and doesn't appear on the album's glossy back cover photo. His credit is limited to "additional guitars," though his composition, "Stucked," is included on the album. He also didn't play on the band's only hit, a nice cover of the Spinners' classic, "I'll Be Around," which reached No. 62 on Billboard's Hot 100. Our guess is that What Is This? was becoming too commercial for Slovak, who preferred the more punky sounds of the Peppers.

Here's Slovak ripping on "My Mind Have Still I," from What Is This?'s Squeezed EP.

How important was Slovak to the Chili Peppers? When Flea asked singer Anthony Kiedis how he felt about Slovak rejoining the Peppers, Kiedis responded, "I'd give my firstborn son to get him back in the band," according to Scar Tissue, his 2004 autobiography.

Slovak was back in the Peppers for the release of their second album, Freaky Styley, produced by legendary funk-master George Clinton. Some have suggested it was on this album that the Peppers came closest to perfecting their trademark punk-funk sound. In this clip from 1985, the band is captured performing "Jungle Man," the opening track from Freaky Styley. Check out Slovak laying down those funky chicken-scratch guitar riffs.

The Chili Peppers don't wear the influence of Jimi Hendrix on their sleeves; rather, Flea wears it on his left bicep in the form of a tattoo of the late guitar god's face. Their amped-up cover of Hendrix's classic "Fire" was a staple of their live set from early on. I was lucky enough to catch them several times early on; including New Year's Eve 1984 at the Club Lingerie in Hollywood — a show hosted by comedienne Sandra Bernhard (whom Flea noted on stage that he wanted to have sex with, just not in that polite of terms). They closed the show that night with "Fire" and Kiedis sent us out with his full moon.

Originally the band released their cover of the song as the B-side on the "Fight Like a Brave" 12-inch single in 1987. However, it didn't appear on album until a version featuring Slovak — and Irons — was released posthumously on 1989's Mother's Milk. Here the band is captured in all their socks-on-their-c--ks, bare-assed glory performing it at the Provinssirock '88 Festival in Seinäjoki, Finland, in Slovak's second-to-last public performance. Check out Slovak going off towards the end of the song, beginning at about the 1:30 mark.