It is not an acronym; it is an abbreviation of *pinnulate* (or *pinnulated*), which means having the pinnulae of a pinnate leaf properly developed.

Gene symbols written in all caps indicate the wild-type condition of the organism: If a gene is referred to as *PIN1*, it means its product (an auxin transporter) permits normal branching (or at lest does not contribute to malformations); when it is written in lower case, *pin1*, it indicates the gene has a variant associated with a mutant (non-pinnate) phenotype.

*PIN1*: nothing is broken

*pin1*: a single stalk without pinnulae (in the case of Arabidopsis)

If the mutant looks like a pin, it is a co-incidence (probably a mnemonic suggested by a English-speaking teacher). The original meaning derives from *pinna*, which refers to the normal appearance leaves and flowers.