Abstract: Introduction of Fas apoptosis inhibitory molecule (FAIM): FAIM was originally discovered in FAS-resistant mouse primary B lymphocytes in 1999, and was thought of as a FAS-apoptosis inhibitor based on overexpression studies (Schneider et al., 1999). FAIM is an approximately 20 kDa intracellular protein, but a subsequent study identified an alternatively spliced form, termed FAIM-Long (L), which has 22 additional amino acids at the N-terminus (Zhong et al., 2001). Thus, the originally identified FAIM was renamed FAIM-Short (S). FAIM is also termed FAIM1 in some publicly available gene/genome databases. Although two other gene products were confusingly termed FAIM2 (also termed lifeguard) and FAIM3 (also termed TOSO), neither FAIM2 nor FAIM3 are related to FAIM-S and FAIM-L in terms of physiological functions or gene/protein homology. FAIM-S is ubiquitously expressed in the body, but FAIM-L is expressed almost exclusively in the brain and testis (Zhong et al., 2001). Overexpression studies using B lymphocytes showed that FAIM inhibits FAS-mediated apoptosis presumably by enhancing NF-κB activation (Schneider et al., 1999; Kaku and Rothstein, 2009a, b). However, the physiological role of FAIM (hereafter FAIM indicates both FAIM-S and FAIM-L) was unknown for many years, partly because no abnormality was detected in FAIM-deficient mice. It was not until our recent studies that we discovered FAIM-deficient cells are more susceptible to cellular stress (Kaku and Rothstein, 2020). In retrospect, its role might have previously been obscured by the lack of stress in vivarium mouse life.