Could be a natural defect, but I don't think it's any kind of cleft, because of the location and the shape.
Looks like a scar. Car accident? Vicious dog? Got thrown through a plate glass window?
She seems to have had it revised by a surgeon in later years.

I recall seeing her (unscarred) in "Up the Creek" and being startled by the scar in "Ghostbusters". Those were released in the same year, and I wondered about how she was injured. So, I think she must have had an accident, as she is unmarked in that first film.
I don't think the cleft palate thing is right because it would have been noticeable prior to "Ghostbusters". For example, Stacy Keach was born with a cleft palate, and his scar has been noticeable throughout his acting career. Also, that's not where Jennifer's scar is positioned. (I always thought that Joaquin/Leaf Phoenix's scar was the result of a cleft palate, but he claims it's a birthmark.)
MystMoonstruck

Jennifer Runyon had a cleft lip I believe. A cleft is a birth defect that occurs when the tissues of the lip and/or palate of the fetus do not fuse very early in pregnancy. A cleft lip, sometimes referred to as a harelip, is an opening in the upper lip that can extend into the base of the nostril. A cleft palate is an opening in the roof of the mouth. The scar is from her surgery as an infant.

This.
Homo No Homo
MAGA
4
L1F3

Where does she live now and how does she look? She was so beautiful back in the day.

This message has been deleted.

She still has the scar, my wife saw her in a store a few days ago
Couldn't quite place her so I posted under "Need to Know Actress with a small scar near lip?"
Two people suggested Jennifer and my wife says yes, that is who she saw!!

Pretty odd. I didn't notice the scar in Charles in Charge. Her appearance in BH90210 was several years later and the scar is very obvious.

I bet she was running with a pen in her mouth. For hundreds of years, mothers have
warned their kids not to do this. I bet she just didn't listen.
I wanna buy your carbon offsets.

She was hot in Ghostbusters!

I just watched 90210 today, and noticed the scar. I wonder what it's from?

I noticed her scar in To all a goodnight (1980) so I agree that later one they used make up to cover it. Just a thought.

She probably didn't have it corrected - it is probably just touched-up by the make-up people before she goes on-camera. They've got thick waxy make-up that fills in most 'craters' on the face. When you meet some of these film 'stars' in person you will be shocked to see things that never seem to show up on-camera.
What's more interesting is in her bio page they've left out her TV appearance with Michael Keaton on Family Ties.

I have tapes of Jennifer Runyon in Charles in Charge and have watched some of them looking for this scar you mention on Jennifer's upper lip. I'm having a little difficulty noticing it. I think I may see the scar, but not totally sure and just checking with you to confirm where this scar is actually located on her upper lip.
I'm not sure if you're working from memory or what, but it appears there's a small indention on her left side of her upper lip. It looks like there's a bit of a small "slicing" motion to it, depending on how she's moving her lips, and what expression she's using. Is that what you're talking about? Sort of looks like she may have poked herself when she was a kid with either a fork or knife. Again, just confirming from you if that's where the scar is located. And if not, if you can point me to where the scar precisely at.
I've seen other people mention a couple of times on a few other sites that Jennifer had a deep scar on one of her cheeks. Again, this is something I've yet to see in the tapes that I have. So I wonder if the scar these people are seeing is the same one you've mentioned, b68except that it's located on her upper lip, and not her cheek.
As far as her having surgery to correct it, I'm not sure.