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What’s mc1r and what do mutations there do?

Mc1r is short for melanocortin 1 receptor, a very important gene/protein in pigment synthesis. In color genetics, it's often called extension gene, too.

The molecule itself basically behaves as a switch: it determines if the cell produces eumelanin (black/brown pigment) or pheomelanin (red/yellow pigment) at any given moment.

In all kind of animals, lots of red, orange, yellow and pale color variants are associated with loss-of-function MC1R mutations, and, on the other hand, melanistic colorations are often caused by gain-of-function mutations. The OMIA database contains documented mutations in 48 different animals, lots of them with several different alleles.

Clown ball phyton, menil fallow deer, blond antartic fur seal, chestnut horse, white camel, red zebu - these are all MC1R-mutants.

Domestic cats however... well, they do have no less but three mutations on this gene, but the standard, wildspread orange that gives us red and tortoiseshell cats is none of them. (That would be on the recently identified Arhgap36 gene, famously sex-linked, unlike MC1R, which is autosomal and inherited independently of sex.) The mc1r cat colors are all relatively new and restricted to one respective breed; they are called amber, russet and carnelian.

Amber tabby norwegian forest cat, (likely solid) russet sepia burmese and carnelian tabby kurilian bobtail.

For further information see my extension tag.