 The study investigates the expression levels of angiotensin-converting enzyme 2, ACE2, in various human tissues to understand the mechanism of severe acute respiratory syndrome coronavirus 2, SARS-CoV-2 infection. ACE2 is a key receptor for SARS-CoV-2 and plays a crucial role in the entry of the virus into human cells. The study found that ACE2 expression levels were highest in the small intestine, testis, kidneys, heart, thyroid, and adipose tissue and lowest in the blood, spleen, bone marrow, brain, blood vessels, and muscle. ACE2 showed medium expression levels in the lungs, colon, liver, bladder, and adrenal gland. The study also found that ACE2 was not differentially expressed between males and females or between younger and older persons in any tissue. However, ACE2 expression levels were positively associated with immune signatures in both males and females in the skin, digestive system, brain, and blood vessels. In the thyroid and lungs, ACE2 expression levels were positively and negatively associated with immune signatures in males and females, respectively, and in the lungs they had a positive and negative correlation in the older and younger groups, respectively. The study provides new insights into the role of ACE2 in the SARS-CoV-2 pandemic and suggests that SARS-CoV-2 may infect other tissues aside from the lungs and infect persons with different sexes, ages, and races equally. This article was authored by Meng Yuan Li, Lin Li, Yu Zhong, and others.