Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic is a genetics pun. A scientist is holding a paper that reads "CAT-GAG" and says "Of course," while another scientist looks on. The caption reads, "The gene sequence for hairballing was beautiful in its simplicity."

The joke works on two levels. In molecular biology, gene sequences are written using the letters A, C, G, and T (representing the four nucleotide bases adenine, cytosine, guanine, and thymine). "CAT-GAG" is a plausible-looking short DNA sequence. But it also reads as two English words: "CAT" (the animal) and "GAG" (the reflex). Since hairballing is what cats do when they gag up a hairball, the gene sequence is "beautifully simple" because it literally spells out what the gene does. The humor comes from the absurd coincidence of a genetic code that is also a perfect plain-English description of its function.