Pichia pastoris is a popular yeast expression system for heterologous protein production. The GC content of the genes to be expressed in Pichia pastoris can vary, but it is generally recommended to have a GC content of around 30-60%. This range is considered suitable for optimal expression in Pichia pastoris.

However, it's important to note that other factors, such as codon usage and sequence elements, can also influence gene expression in this system. Optimization of these factors, along with the GC content, is often performed to enhance protein production efficiency in Pichia pastoris. Researchers may use bioinformatics tools to analyze and optimize the coding sequences for expression in the specific host organism.