What Causes Mass In One Kidney And Multiple Cysts In Other?

my 7 yr old son has multiply cyst on his right kidney and last year while having a ultrasound, the dr came across a mass on his left kidney.he has had two mri to see if the mass has grown but it has not. he just went through gentic bloodwork for testing for tsc and all the test came back negative. now the dr wants my son to go through more gentic testing. what else could it be??