Familial isolated pituitary adenoma (FIPA) occurs in families and is unrelated 
to multiple endocrine neoplasia type 1 and Carney complex. Mutations in AIP 
account only for 15-25% of FIPA families. CDKN1B mutations cause MEN4 in which 
affected patients can suffer from pituitary adenomas. With this study, we wanted 
to assess whether mutations in CDKN1B occur among a large cohort of AIP 
mutation-negative FIPA kindreds. Eighty-eight AIP mutation-negative FIPA 
families were studied and 124 affected subjects underwent sequencing of CDKN1B. 
Functional analysis of putative CDKN1B mutations was performed using in silico 
and in vitro approaches. Germline CDKN1B analysis revealed two nucleotide 
changes: c.286A>C (p.K96Q) and c.356T>C (p.I119T). In vitro, the K96Q change 
decreased p27 affinity for Grb2 but did not segregate with pituitary adenoma in 
the FIPA kindred. The I119T substitution occurred in a female patient with 
acromegaly. p27(I119T) shows an abnormal migration pattern by SDS-PAGE. Three 
variants (p.S56T, p.T142T, and c.605+36C>T) are likely nonpathogenic because In 
vitro effects were not seen. In conclusion, two patients had germline sequence 
changes in CDKN1B, which led to functional alterations in the encoded p27 
proteins in vitro. Such rare CDKN1B variants may contribute to the development 
of pituitary adenomas, but their low incidence and lack of clear segregation 
with affected patients make CDKN1B sequencing unlikely to be of use in routine 
genetic investigation of FIPA kindreds. However, further characterization of the 
role of CDKN1B in pituitary tumorigenesis in these and other cases could help 
clarify the clinicopathological profile of MEN4.