Which of these is not true?

Options

A. Actin filaments are in muscle cells
B. Motor molecules use microtubules as tracts
C. Intermediate filaments sometimes contain keratin
D. Microtubules radiate out from the ER

Explanation

To determine which statement is not true among the given options, let's analyze each one step by step:

### Step 1: Evaluate the first option
**Statement:** Actin filaments are in muscle cells.
**Analysis:** This statement is true. Muscle cells contain myofibrils, which are made up of two types of filaments: actin (thin filaments) and myosin (thick filaments). Therefore, actin filaments are indeed present in muscle cells.

### Step 2: Evaluate the second option
**Statement:** Motor molecules use microtubules as tracks.
**Analysis:** This statement is also true. Motor proteins such as kinesin and dynein move along microtubules, which serve as tracks for intracellular transport. Hence, this option is correct.

### Step 3: Evaluate the third option
**Statement:** Intermediate filaments sometimes contain keratin.
**Analysis:** This statement is true as well. Intermediate filaments are composed of various proteins, including keratin, which is a type of fibrous protein found in many cells. Therefore, this option is correct.

### Step 4: Evaluate the fourth option
**Statement:** Microtubules radiate out from the endoplasmic reticulum.
**Analysis:** This statement is not true. Microtubules actually radiate from the centrosome (also known as the microtubule organizing center) towards the plasma membrane, not from the endoplasmic reticulum. Therefore, this option is incorrect.

### Conclusion
The statement that is not true is: **"Microtubules radiate out from the endoplasmic reticulum."**

### Final Answer
The correct answer is option number 4.

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