In this study, the investigators will examine the relationship between a patient's genes (DNA), or "genotype", and how the patient's body breaks down and removes or "metabolizes" the anti-cancer drug irinotecan. Circulating blood level of irinotecan plays an important role in how well this drug works against a patient's cancer as well as the adverse side effects the patient may experience. The current standard dose of irinotecan was determined in clinical trials without knowing individual genotypes and thus does not take into account a patient's ability to metabolize irinotecan. This means that based on one genotype the current standard dose of irinotecan may be correct or based on other genotypes the standard dose could result in lower and possibly less effective blood levels and result in significant under-dosing of irinotecan.