Asthma-COPD overlap Syndrome (ACOS) is a syndrome in which a patient suffers from both chronic obstructive pulmonary disease (COPD) and asthma. Standard definition and diagnostic criteria of this overlap syndrome is not established yet. It is important to formulate a definition of ACOS for accurate diagnosis and studies. Few biomarkers have been found which can be useful for diagnosis but much research is needed. The prevalence of ACOS depends upon the definition, diagnosis criteria and population analyzed. Different single nucleotide polymorphism (SNPs) has been identified in limited population.