Steroid hormones such as estradiol and testosterone are built on a shared hydrocarbon skeleton. How is that skeleton described?

1. Steroid hormones are an important class of small hydrophobic ligands. 2. Steroids are lipids that have a hydrocarbon skeleton with four fused rings. 3. Different steroids have different functional groups attached to this same carbon skeleton. 4. A single unbranched chain or a phosphate-linked structure does not match the four-fused-ring description given for steroids. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 9 "Cell Communication", section Signaling Molecules_