Minority racism is sometimes considered controversial because of theories of power in society. Some theories of racism insist that racism can only exist in the context of social power so it can be imposed upon others. Yet discrimination and racism has also been noted between racially marginalized groups. For example, there has been ongoing violence between African American and Mexican American gangs, particularly in Southern California. There have been reports of racially motivated attacks against Mexican Americans who have moved into neighborhoods occupied mostly by African Americans, and vice versa. According to gang experts and law enforcement agents, a longstanding race war between the Mexican Mafia and the Black Guerilla Family, a rival African American prison gang, has generated such intense racial hatred among Mexican Mafia leaders, or shot callers, that they have issued a "green light" on all blacks. This amounts to a standing authorization for Latino gang members to prove their mettle by terrorizing or even murdering any blacks sighted in a neighborhood claimed by a gang loyal to the Mexican Mafia. There have been several significant riots in California prisons where Mexican American inmates and African Americans have targeted each other particularly, based on racial reasons.