Class I PI 3-kinases are a subgroup of the enzyme family, phosphoinositide 3-kinase that possess a common protein domain structure, substrate specificity, and method of activation. Class I PI 3-kinases are further divided into two subclasses, class IA PI 3-kinases and class IB PI 3-kinases.

Class IA PI 3-kinases

Class IA PI 3-kinases are activated by receptor tyrosine kinases (RTKs).

There are three catalytic subunits that are classified as class IA PI 3-kinases:^{[citation needed]}

• p110α
• p110β
• p110δ

There are currently five regulatory subunits that are known to associate with class IA PI 3-kinases catalytic subunits:^{[citation needed]}

• p85α and p85β
• p55α and p55γ
• p50α

Class IB PI 3-kinases

Class IB PI 3-kinases are activated by G-protein-coupled receptors (GPCRs).

The only known class IB PI 3-kinase catalytic subunit is p110γ.

There are two known regulatory subunits for p110γ:

• p101
• p84/ p87PIKAP.