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What is Simian crease is seen in: and explain in detail?
Simian crease is an older name for a condition now more commonly called single transverse palmar crease. It refers to a single crease across the palm of the hand. In most individuals, there are two slightly oﬀset creases across the palm. Simian crease is seen in:

Down's syndrome
Trisomy of chromosome No.13 
Cri-du-chat syndrome