This article is from the NYPD Blue FAQ, by Dave Chapman, Alan Sepinwall and Jeff Knapp with numerous contributions by others.
It seems like they've had more than Murphy Brown has had
secretaries... it seems like there is always discussion the
group about "the new PAA". With the anticipated fifth season
return of John Irvin, perhaps that is a trend that will end...
In the beginning, there was no special PAA, just a string of
faceless people who sat at the desk and did less than any
other PAA since. The PAA was not a part of the squad then...
until Donna Abandando.
Donna has the best track record of any PAA the squad has had,
lasting two seasons (Starting with "Tempest in a C-Cup" (Ep.
1.8) and ending with "A Death in the Family" (Ep 3.20)
From there we have a bunch of people often not lasting more
than an episode or two (brackets are their PAA tenure):
Upstairs John (Eps. 2.17 - 2.20 ) took over for a
romantically recovering Donna, and was the best temp PAA
the squad had. When Donna came back, Fancy got John a
job upstairs in Anti-Crime which coined the nickname
"Upstairs John" John left anti-crime in Ep 3.22 to go
back to 1 Police Plaza (and the abysmal Bochco-sitcom
Public Morals), opening the door for Gina/Geri to take
Lucy Kinley (Eps. 3.21, 3.22) gets transferred for
Geri Turner (Eps. 4.01 - 4.06) gets transferred upstairs
after making Andy very uncomfortable (not to mention
being a little psycho - which has nothing to do with her
Gina Colon (Eps. 4.06 - 5.03) worked for Anri-Crime at
the beginning of the season and caught James' eye.
Imagine James' thrill when Fancy worked out a trade for
the not-working-out Geri. James liked her so much, he
married her... and put her "in a family way" so she's on
maternity leave from the squad. I'll be surprised if she
ever comes back as a PAA and not as Mrs. Martinez.
Naomi Reynolds (5.04 - 5.12) appeared to be a southern
belle until her landlord cheesed her off and she started
berating him in her native Australian accent. This lead
to problems with INS (and the kookiest subplot involving
a PAA ever - beating Geri's "I'm into rubber" schtick,
IMHO) and her leaving the squad.
Dolores Mayo (5.13 - ??) is a chain smoking looker who
spends more time downstairs than she does at her desk.
How she's leaving is anyone's guess.