A sad story to begin with when Mark St. John had to leave KISS (or was forced to leave) because of an arthritic problem in 1984 and now this.
A very good guitar player.
I'm a huge KISS fan and while I despised the hair metal era, Animalize was a pretty good effort.
I'm sure CBK and other KISS fans are saddened by this news.
From Blabbermouth.net:

KNAC.COM is reporting that former KISS guitarist Mark St. John (photo) died this morning from a cerebral hemorrhage. He was 51 years old.

Acccording to KissFAQ.com, Mark St. John was born Mark Norton, in Hollywood, California, on February 7, 1956. His parents moved in the early 1960s to Garden Grove, CA where Mark was initially more interested in sports than music. In an interview with KISS Strike, Mark recalled, "I played all sports, in fact I played sports before I got into music. Especially basketball, I mean I tried all sports but basketball is what I excelled in. And when I got into high school we started playing CIF with other teams from L.A. and they were all black and I felt that I wasn't a very good basketball player. I mean they just slip and slide all over you, they jump higher and they run faster and you felt belittled after the first five minutes and I gave it up after that."

Growing up in Southern California, Mark didn't start playing guitar until he was a sophomore in highschool in 1972 when he also started to grow his hair longer, undoubtedly part of that age-group's rebellion from their parents and society. Initially guitar playing started as a hobby, but within a year Mark had become quite serious about it.

As to why he started plaing guitar, Mark recalled, "I guess because the guitar was the new thing happening — it was the sound. I'm from a generation that went through a whole different thing in music, like a culture-type thing. It was the voice of the people. It was a source of rebellion and the guitar was one of the biggest things. I remember watching television and seeing THE BEATLES and THE ROLLING STONES on the 'Ed Sullivan Show'. They didn't have tubas, they had guitars and everyone was going crazy and that put a spark in my mind... The guitar is rock 'n roll. That's what it's all about. From the '50s on, the emphasis was on the guitar sound. Everyone was trying to make the guitar sound like other instruments, but it's not so easy to make other instruments sound like the guitar. The guitar is what's happening"

Mark has a brother, Michael (born 1964), who plays bass. The two would be in bands together following Mark's tenure in KISS. They also have one sister (born 1960).

I actually got to meet Mark St. John at a KISS convention eight years ago. He was very likable, friendly and had some interesting stories to tell. This is very sad news.

I used to play Animalize daily when it first came out - it just blew me away.

KISS put out some decent stuff in the 80s. That's very sad news.