S-Cells (S細胞 Esu Saibō) are microscopic cells found in most Saiyans.
The amount of S-Cells depends on the spirit of the Saiyan in question: the more gentle the spirit, the greater the amount of S-Cells; alternatively, when a Saiyan increases their battle power, it can also increase the amount of S-Cells in their body. S-Cells are essential to becoming a Super Saiyan, and allows a Saiyan with a certain amount of them to transform by feeling intense levels of anger or sadness. Alternatively, by focusing their power into a tingling sensation in their upper-back (directly below their neck) the Saiyan in question can trigger the transformation into a Super Saiyan without the need for emotional stress.
The lack of understanding the role S-Cells play in the Super Saiyan transformation has given raise to the misconception that a Saiyan must be "pure of heart", something that both Tien Shinhan and Vegeta assumed to be true.
Gentle Saiyans having more S-Cells explains why Super Saiyans were rare for a long time in Universe 7, as following the genocide of the good Saiyans, the Saiyans who remained lived in a violent society that valued aggression and ruthlessness.