Involved in the control of stem cells and sex determination in the C.elegans hermaphrodite germline (PubMed:9393998). May also play a role in the hermaphrodite germline proliferation and oogenesis (PubMed:15454534, PubMed:27864381). By binding to the 3'-UTR, represses phosphatase lip-1 expression in the distal part of the germline mitotic zone (PubMed:16319922). Binds specifically to the regulatory region of fem-3 3'-UTR and mediates the sperm/oocyte switch (PubMed:9393998). Negatively regulates gld-3 expression possibly by directly binding to two sites within the gld-3 isoform b 3'-UTR (PubMed:15454534). Suppresses germline tumor formation by preventing the dedifferentiation of secondary spermatocytes (PubMed:22820175). Interacts (via C-terminus) with gld-3 isoform A in an RNA-independent manner (PubMed:12431376). Interacts with dlc-1, and is required for the localization of fbf-2 to P granules (PubMed:27864381). Recruited to P granules by dlc-1. Expressed specifically in the germline (at protein level). Expression increases in abundance during postembryonic development and peaks during the fourth larval stage. Fbf-1 and fbf-2 are >90% identical on both nucleotide and protein sequence level. Experimental approaches often do not distinguish between the two genes, which are collectively referred to as fbf and are considered to be functionally redundant.