diff --git a/src/__tests__/fillRequestObject.spec.ts b/src/__tests__/fillRequestObject.spec.ts
new file mode 100644
index 0000000..a9c29da
--- /dev/null
+++ b/src/__tests__/fillRequestObject.spec.ts
@@ -0,0 +1,59 @@
+import Client from '../internal';
+import { CheckParametersTypes } from '../internal/internal.types';
+
+it('should return an object with input type, input set, and logical operator when only the required arguments are provided', () => {
+  const inputType: string = 'cell';
+  const outputType: string = 'organ';
+  const inputSet: string[] = ['fake'];
+
+  const testObj: CheckParametersTypes = {
+    inputType,
+    outputType,
+    inputSet,
+  };
+  expect(Client.fillRequestObject(testObj)).toEqual({
+    input_type: inputType,
+    input_set: inputSet,
+    logical_operator: 'and',
+  });
+});
+
+it('should return an object with a p value when input type is organ or gene and output type is gene', () => {
+  const inputType: string = 'organ';
+  const outputType: string = 'gene';
+  const inputSet: string[] = ['fake'];
+  const pValue: number = 0.4;
+
+  const testObj: CheckParametersTypes = {
+    inputType,
+    outputType,
+    pValue,
+    inputSet,
+  };
+  expect(Client.fillRequestObject(testObj)).toEqual({
+    input_type: inputType,
+    input_set: inputSet,
+    p_value: pValue,
+    logical_operator: 'and',
+  });
+});
+
+it('should return an object with a genomic modality if it is defined', () => {
+  const inputType: string = 'organ';
+  const outputType: string = 'cell';
+  const inputSet: string[] = ['fake'];
+  const genomicModality: string = 'fake';
+
+  const testObj: CheckParametersTypes = {
+    inputType,
+    outputType,
+    genomicModality,
+    inputSet,
+  };
+  expect(Client.fillRequestObject(testObj)).toEqual({
+    input_type: inputType,
+    input_set: inputSet,
+    genomic_modality: genomicModality,
+    logical_operator: 'and',
+  });
+});
diff --git a/src/internal/index.ts b/src/internal/index.ts
index 8bcccec..bfd4d2f 100644
--- a/src/internal/index.ts
+++ b/src/internal/index.ts
@@ -1,4 +1,4 @@
-import { CheckParametersTypes } from './internal.types';
+import { CheckParametersTypes, RequestObjectParametersTypes, RequestObjectTypes } from './internal.types';
 import { outputTypes, inputTypes, genomicModalities } from './internal.config';
 
 class Client {
@@ -37,9 +37,27 @@ class Client {
     }
   }
 
-  /*
-  fillRequestObject() {}
+  static fillRequestObject({
+    inputType,
+    outputType,
+    inputSet,
+    genomicModality,
+    pValue,
+  }: RequestObjectParametersTypes): RequestObjectTypes {
+    const requestObject = { input_type: inputType, input_set: inputSet };
+
+    if (['organ', 'gene'].includes(inputType) && outputType === 'gene') {
+      requestObject.p_value = pValue;
+    }
+
+    if (genomicModality) {
+      requestObject.genomic_modality = genomicModality;
+    }
 
+    requestObject.logical_operator = 'and';
+    return requestObject;
+  }
+  /*
   hubmapQuery() {}
 
   setIntersection() {}
diff --git a/src/internal/internal.types.ts b/src/internal/internal.types.ts
index d072c14..1ed95ad 100644
--- a/src/internal/internal.types.ts
+++ b/src/internal/internal.types.ts
@@ -8,9 +8,26 @@ interface InputTypes {
 interface CheckParametersTypes {
   inputType: string;
   outputType: keyof InputTypes;
-  // inputSet: string[];
-  genomicModality: string;
+  inputSet?: string[];
+  genomicModality?: string;
   pValue?: number;
 }
 
-export { InputTypes, CheckParametersTypes };
+interface RequestObjectParametersTypes {
+  inputType: string;
+  outputType: keyof InputTypes;
+  inputSet: string[];
+  genomicModality?: string;
+  pValue?: number;
+}
+interface RequestObjectTypes {
+  /* eslint-disable camelcase */
+  input_type: string;
+  input_set: string[];
+  genomic_modality?: string;
+  p_value?: number;
+  logical_operator: 'and';
+  /* eslint-enable camelcase */
+}
+
+export { InputTypes, CheckParametersTypes, RequestObjectParametersTypes, RequestObjectTypes };