The purpose of this study is to understand how an experimental drug called NTLA-2001 affects a heart condition called transthyretin (TTR) amyloidosis with cardiomyopathy (ATTR-CM), a condition in which the heart muscle does not pump blood through the body as it should due to build-up of abnormal protein (TTR) between the muscle fibers in the heart. The study aims to determine whether NTLA-2001 is safe and effective in people with ATTR-CM. NTLA-2001 uses a special gene-editing tool to change an individual’s DNA and turn off the TTR gene in liver cells. This stops the TTR gene from making the TTR protein, which may help improve the symptoms of ATTR-CM. (The process does not add any new permanent genetic material to the body; it just changes the way the patient’s DNA works to stop producing the harmful TTR protein.) Participants will be assigned to receive either NTLA-2001 or placebo (fluids that contain no active ingredients). For every 3 participants in the study, a computer will randomly choose 2 people to receive NTLA-2001 and 1 person to receive placebo. NTLA-2001 is designed to work only in the liver, where the TTR protein is made.