Definition: Given an abstract of a paper, generate a title for this paper such that conveys the key focus of the paper.
Input: Abstract: Proliferin (PLF) is a member of the PRL-GH family whose mRNA has been detected in proliferating mouse cells in culture and in mouse placenta. Suspensions of murine placental tissue have been shown to produce PLF detectable by immunoprecipitation. Here we report that PLF is secreted as a heterogeneous glycoprotein by growing mouse cell lines and placental tissue and that the first 29 amino acids of the preprotein serve as a signal peptide. Placental PLF is immunologically distinguishable from the 2 previously described mouse placental lactogens.

Secretion of proliferin.