This highlight focuses on the marine natural products that target cytoskeletal proteins and modulate protein–protein interactions in unique fashions.

A new 9,11‐secosteroid having an unprecedented tricyclic γ‐diketone structure, aplysiasecosterol A (1), was isolated from the sea hare Aplysia kurodai. The structure was determined by one‐ and two‐dimensional NMR spectroscopic analysis, molecular modeling studies, a comparison of experimental and calculated ECD spectra, and a modified Mosher′s method. Aplysiasecosterol A (1) exhibited cytotoxicity against human myelocytic leukemia HL‐60 cells. A biosynthetic pathway for 1 from a known cholesterol was proposed and includes twice α‐ketol rearrangements and an intramolecular acetalization.