Autoimmune diseases like rheumatoid arthritis are multifactorial in nature, 
requiring both genetic and environmental factors for onset. Increased 
predisposition of females to a wide range of autoimmune diseases points to a 
gender bias in the multifactorial etiology of these disorders. However, the 
existing evidence to date has not provided any conclusive mechanism of 
gender-bias beyond the role of hormones and sex chromosomes. The gut microbiome, 
which impacts the innate and adaptive branches of immunity, not only influences 
the development of autoimmune disorders but may interact with sex-hormones to 
modulate disease progression and sex-bias. Here, we review the current 
information on gender bias in autoimmunity and discuss the potential of 
microbiome-derived biomarkers to help unravel the complex interplay between 
genes, environment and hormones in rheumatoid arthritis.